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THE COMPARATIVE AGRICULTURAL VALUE OF INSOL- 
UBLE MINERAL PHOSPHATES OF ALUMINUM, 
IRON AND CALCIUM 



BY 

JACOBUS STEPIIANUS MARAIS 



University of Illinois 



Reprinted prom 

Soil Science, Vol. XIII, No. 5, May, 1922 



Digitized by the Internet Archive 
in 2010 with funding from 
The Library of Congress 



http://www.archive.org/details/connparativeagricOOnnara 



THE COMPARATIVE AGRICULTURAL VALUE OF 

INSOLUBLE MINERAL PHOSPHATES OF 

ALUMINUM, IRON, AND CALCIUM 



BY 

JACOBUS STEPHANUS MARAIS 

Pass B.A. University of Cape of Good Hope, 1916 
Honors B.A. University of Cape of Good Hope, 1917 



THESIS 

Submitted in Partial Fulfillment of the Requirements for the 

Degree of 

DOCTOR OF PHILOSOPHY 

In Agronomy 

IN 

THE GRADUATE SCHOOL 

of the 

UNIVERSITY OF ILLINOIS 
1921 






H' W g'!' '' ' ^ '•'• i ' !n t^tU » < '-t 



mmm of conq«ess 



DOCUIMjSNTS 



Repiinted from Soil Science 
Vol . XIII, No. 5, May, 1922 



NJ- 



THE COMPARATIVE AGRICULTURAL VALUE OF INSOLUBLE 
MINERAL PHOSPHATES OF ALUMINUM, IRON, 
AND CALCIUMi 

JACOBUS STEPHANUS MARAIS 

University of Illinois 

Received for publication June 6, 1921 
INTRODUCTION 

"Phosphorus is the only element that must be purchased and returned to 
the most common soils of the United States. Phosphorus is the key to per- 
manent agriculture on these lands." This statement of C. G. Hopkins (29) 
emphasizes the extreme importance of the phosphorus problem in modern 
agriculture; especially at the present time when the seriousness of the world 
food situation is making an urgent appeal to agriculturists to increase and to 
maintain permanently the fertility of all tillable soils. 

The acute shortage of transportation facilities has placed farmers, not 
conveniently situated near phosphate-producing centres, at a disadvantage 
with regard to procuring phosphorus at other than exorbitant prices. This 
has resulted in a world-wide prospecting for phosphate deposits and has caused 
considerable speculation as to the feasibility of utilizing iron and aluminum 
phosphates for agricultural purposes. 

In spite of the fact that a considerable amount of work had been done that 
demonstrates the value of aluminum and iron phosphates, the general belief 
is that they have little significance from an agricultural point of view. The 
fact that they are practically useless for acid phosphate manufacture, com- 
bined with their low solubility in citric acid and ammonium citrate solutions 
is probably the main cause for the popular conception of their agricultural 
value. 

There are also numerous statements by eminent scientists scattered through- 
out the literature in which aluminum and iron phosphates are referred to as 
being particularly unavailable as plant-food. The fleeting action of super- 

1 Thesis submitted in partial fulfillment of the requirements for the degree of Doctor of 
Philosophy in Agronomy in the Graduate School of the University of Illinois. The author 
wishes to express his obligations to Prof. Robert Stewart for valuable suggestions and criti- 
cisms when the experiments were first planned; to Prof. A. L. Whiting and Dr. E. E. De 
Turk for help and advice during the progress of the later experiments and for reading the 
manuscript; and to Messrs. J. C. Anderson and W. Green for assistance rendered in the 
greenhouse in preparation and care of the pot cultures. 

355 

SOIL SCIENCE, VOL. XIII, NO. 5 



356 JACOBUS STEPHANUS MARAIS 

phosphates on soils rich in aluminum and iron oxides, for example, is ascribed 
to the conversion of this phosphate into aluminum and iron phosphates. 

It was the object of the experiments reported in this paper to determine the 
comparative values of various phosphates of aluminum, iron and calcium which 
occur in nature, and simultaneously to determine how they are affected by 
diverse collateral treatments. 

REVIEW OF LITERATURE 
Some fundamental considerations 

A fundamental fact, which has a very important bearing on the phosphate problem in 
soils was brought to light by the work of Schloesing and Kossovitsch. In 1899, Schloesing 
(67) demonstrated the fact that plants can obtain their phosphorus from very dilute solutions, 
solutions containing only 1 to 2 mgm. phosphoric anhydride per liter. This emphasizes 
the importance of naturally dissolved phosphates in the soil solution for plant nutrition. 
Kossovitsch (37) repeated these experiments, verified Schloesing's results and showed simul- 
taneously that the relative feeding powers of plants do not rest solely on their ability to 
utilize the phosphorus occurring in dilute solutions. Flax, when compared with mustard 
and peas, has but feeble powers to utilize the phosphorus of tricalcium phosphate rock, but 
was shown to make good growth on a nutrient solution, which contained only 1 . 3 mgm. of 
phosphoric anhydride per liter. 

From the work of Schloesing one might at first conclude that the plant roots exert a solvent 
action on phosphates. Sachs (65) in 1860, demonstrated that plants roots were capable of 
corroding marble plates. In 1896, Czapek (13) conducted extensive investigations to deter- 
mine whether roots excrete or secrete acids, which might function in dissolving plant-food. 
Eventually he concluded that carbonic acid was the only acid given off in considerable quan- 
tity by live roots of plants. In 1902, Kossovitsch (37) demonstrated clearly that the plant 
roots themselves and not the nutrient solution were responsible for obtaining phosphorus 
from phosphorite. The following device was employed by him to determine this factor: 
Plants were grown in two sets of cylinders. In the one set, sand mixed with tricalcium phos- 
phate was used as a medium for the plants to grow in. Five liters of nutrient solution were 
passed daily through each cylinder. In the second set, pure sand was used as a medium for 
growth. As in the above case, five liters of nutrient solution were added daily with the excep- 
tion that the nutrient solution was first made to pass through another cylinder containing a 
mixture of quartz sand and tricalcium phosphate and in which no plants were growing. If 
the nutrient solution acted as a solvent of the phosphate, the plants in the second set of 
cylinders should have made a fair growth. The plants grew well in the first set and made 
hardly any growth in the second, proving that if the nutrient solution exerted any solvent 
action on the tricalcium phosphate, its action was very slight and that the action of the roots 
themselves was a very much more important factor. In 1911, Prianishnikov (61) made the 
claim that iron and aluminum phosphates were gradually decomposed by water and that root 
excretions do not play the important r6le in assimilation of these phosphates that has usually 
been ascribed to them. 

Varying ability of plants to assimilate phosphorus from insoluble phosphates 

In 1893, Balentine (3) working at the Maine Agricultural Experiment Station reported 
that Graminae were benefited more by acid phosphate than by redondite and rock phosphate, 
and that plants of the Cruciferae family were especially strong feeders on rock phosphate. 
Two years later, Merrill and Jordan (42) placed the four botanical families studied in the order 
given below as regards their foraging powers for insoluble phosphates. 



AGRICULTURAL VALUE OF INSOLUBLE MINERAL PHOSPHATES 357 

1. Leguminosae as represented by peas and clover. 

2. Cruciferae as represented by turnips and ruta-bagas. 

3. Graminae as represented by barley and corn. 

4. Solanaceae as represented by tomatoes and potatoes. 

The insoluble phosphates employed in this investigation were Florida rock phosphate, iron 
phosphate, and aluminum phosphate. 

Kossovitsch at various times between 1898 and 1910 made mention in his writings concern- 
ing the feeding powers of different species of plants. In 1901 (36), he commented on the 
strong feeding powers of buckwheat and mustard when grown with phosphorite as a source 
of phosphorus. In a later publication (39) in which he summarized his work on the utiliza- 
tion of phosphorite by mustard, clover, oats, and flax, he placed these plants in the order in 
which they are here mentioned as regards their powers to utilize phosphorite. It should be 
observed that this order is somewhat similar to that put forth by Merrill and Jordan. Kosso- 
vitsch (38) also tried to correlate the feeding powers of plants with their ability to excrete 
carbonic acid, but the difference in the amounts excreted did not Justify the drawing of any 
definite conclusions. 

Schreiber (68) experimented with eleven species of the Graminae, nine of the Leguminosae, 
three of the Cruciferae, and eleven miscellaneous plants. The Leguminosae, the Cruciferae, 
and buckwheat utilized mineral phosphates to a considerable extent, whereas the Graminae, 
flax, tobacco, carrots, asparagus, beets, and potatoes showed little solvent powers. 

Wheeler and Adams of Rhode Island (82, 83), Prianishnikov (56, 57), Bonomi (5), Ged- 
roits (23), Chirikov (Tschirikov) (11), Semushkin (69), and Soderbaum (72), are among 
other workers who have drawn attention to the individuality of plants with respect to the 
topic under discussion. In nearly all these cases, their results agree in a general way with 
those of Merrill and Jordan. The work of the above investigators will be considered later 
in connection with another phase of our problem. 

Emil Truog (76, 77) has propounded a theory to explain the individuality of plants with 
regard to their feeding powers. Plants with a high calcium content he stated, have a rela- 
tively high feeding power for the phosphorus in phosphorites. For plants with relatively 
low calcium content, the reverse is true. Clover, alfalfa, peas, buckwheat, and several of 
the Cruciferae have high calcium content and are, therefore, according to this theory, powerful 
feeders on insoluble phosphates. Corn, rye, oats, wheat, and millet fall in the opposite class. 
A calcium oxide content of less than 1 per cent may be considered low. In another publica- 
tion (78), Truog claimed that high internal acidity of roots is accompanied by high feeding 
powers for calcium. Logically then, plants with roots of high internal acidity are capable 
of utilizing insoluble phosphates with greater success than plants with roots of relatively lower 
internal acidity. It is clear tliat the individuality of the plants is a large factor when the 
availability of phosphates is being considered. 

Effect of soil on availability of insoluble phosphates 

In studying this question three characteristics of soil have been considered by workers: 

1. Mechanical composition. 

2 . Amount of organic matter in soil. 

3. Reaction of the soil. 

It is generally held (41) that it is preferable to use bone meal and basic slag on warm sandy 
soils. Soluble phosphates are put to better use on heavier clay soils. Wheeler and Adams 
(83) claimed that the addition of three-fourths to one ton of limestone per acre removes the 
drawback of using soluble phosphates on light sandy soils. On peat and muck soils, the 
first applications of soluble phosphates are ineffective, due to their entering into colloidal 
combinations, but after these demands have been met, their effects are noticeable. Con- 
cerning the reaction, predominant opinion asserts that soluble phosphates are employed with 
the greatest success on calcareous soils (14, 27, 49). Hilgard (27) in his celebrated work, 



358 JACOBUS STEPHANUS MARAIS 

"Soils," made the following statement, ". . . .in the presence of high lime per- 
centages, relatively low percentages of phosphoric acid and potash may nevertheless prove 
adequate; while the same or even higher amounts, in the absence of satisfactory lime per- 
centages, prove insufficient for good production." Paturel (49), Deherain (14) and others 
claimed that unless sufficient lime be present, the phosphoric acid is fixed by aluminum and 
iron oxides into unavailable combinations. On the other hand, this view appears contra- 
dictory to the observations of Schloesing, fils, regarding the solubility of phosphoric acid in 
the presence of carbonate of lime (66), but natural conditions seem fully to justify Hilgard's 
conclusions. Numerous investigators found aluminum phosphates to be verj' beneficial to 
plant growth provided they were employed on soils well supplied with lime. Results in Mary- 
land (50), France (1), and Rhode Island (82, 83) all show that favorable results with alumi- 
num phosphate have always been obtained when the phosphate is used in connection with 
lime or on soils naturally calcareous. When tricalcium phosphate is employed, the best 
immediate results seem to be obtained on soils not saturated with bases (24) or on soils well 
supplied with organic matter (28, 82). 

Effect of nitrogen compounds on availability of insoluble phosphates 

Prianishnikov and a large number of other Russian workers have studied very carefully 
the effect of various nitrogen compounds on the availability of insoluble phosphates. All 
the results agree in general that ammonium sulfate enhances the availability of insoluble 
phosphates and that ammonium nitrate likewise increases the availability, but to a lesser 
extent. Sodium nitrate either has no effect or depresses the availability. Calcium nitrate 
is similar in its effect to sodium nitrate, but less marked. These results are due to inher- 
ent properties of the salts themselves and not to their conversion into other compounds, for 
example the formation of nitric acid as the result of nitrification of ammonium salts. Kosso- 
vitsch (36) was responsible for the classic work in regard to the effect of ammonium salts. 
In experiments in which the possibility of nitrification being a factor was carefully prevented, 
he confirmed in all instances the deductions of Prianishnikov. Wheeler and Adams (83) 
commenting upon Warington's work (81) seem to be of the opinion that with aluminum 
phosphates results would have been established which would be the reverse of those given 
above. The fact that nitrification materially affects the availability of insoluble phosphates 
has been definitely established by the investigations of Hopkins and Whiting (30). Soder- 
baum (72) checked up Prianishnikov's deductions. He believed that the physiological reac- 
tion of the accompanying nitrogenous fertilizer plays an important part, but claimed that other 
factors, such as kind of plant, soU and other collateral treatments used, may lessen or even 
reverse the influence of this factor. This point is well brought out by Chirikov (10) who found 
that when calcium nitrate replaced ammonium sulfate in his buckwheat cultures, the yields 
were not reduced, but increased. Nedokuchaev (46) working with different crops, oats and 
flax, reported that yields were lower where calcium nitrate was used in lieu of ammonium 
sulfate. On the whole Prianishnikov's deductions seem to be accurate, but we should bear 
in mind that no hard and fast rule can be laid down. In work on the availability of phos- 
fates, the accompanying nitrogenous fertilizer is a factor that must be remembered, espe- 
cially when we attempt to make generalizations from our results. 

Effect of lime on availability of insoluble phosphates 

When the effect on the availability of insoluble phosphates was considered, the influence 
of lime came up for discussion since the reaction of the soil and lime content of the soil are 
closely interrelated. Some further opinions on the effect of lime follows. Prianishnikov 
(61) divided the phosphates into two groups; the one, including tricalcium phosphate, bone 
meal, and phosphorite, consists of those of which the assimilation is markedly reduced by the 
lime; the other, including acid phosphate (mono- and di-calcium phosphates), Thomas slag, 



AGRICULTURAL VALUE OF INSOLUBLE MINERAL PHOSPHATES 359 

mono-potassium phosphate, iron phosphate, and aluminum phosphate, consists of those 
unaffected by the addition of lime or even benefited by it. The studies were made in sand 
cultures. The crops employed were barley, peas, oats, wheat, and buckwheat. In all cases, 
however, where ammonium nitrogen was substituted for nitrate nitrogen, liming was bene- 
ficial. Shulov (71) in studies similar to those of Prianishnikov, determined that the assimil- 
ability of pure ferrous phosphate and vivianite was unaffected by lime; that of tricalcium 
phosphate, in the forms of bone meal and phosphorite, was adversely affected; and that of 
superphosphate, precipitated phosphate and Thomas slag was only slightly reduced. Gaither 
(21) explained the lack of harmful effects of lime upon the availability of soil phosphates as 
due to its action in replacing iron and aluminum in combination with phosphorus and so 
rendering the phosphates more soluble. Gaither used0.2iV nitric acid as a solvent for 
determining available phosphorus. Wheeler and Adams (84) pointed out that, in the phos- 
phate experiments at Rhode Island, iron and aluminum phosphates were more efficient than 
floats on limed land. This agrees with the findings of Prianishnikov. 

Effect of various solvents on the availability ofijisoluble phosphates 

It is beyond the scope of this work to enter into the controversy as to which solvents of 
phosphates can be used for determining their availability to plants. Some literature which 
has a bearing on this work is quoted. Risler (64) claimed that carbonic acid has much less 
solvent action on aluminum and iron phosphates than on calcium phosphates. Wagner 
(80) and later Storer (73) claimed that alkalies, such as sodium carbonate, ammonium carbo- 
nate, etc., can dissolve phosphates of iron and aluminum. Cameron and Bell (9) claimed 
to have proved that soil phosphates are decomposed or hydrolyzed by water with formation- 
of other phosphates containing relatively more of the base. Zecchini (85) reported that alu 
minum and iron phosphates are very insoluble except in alkaline solution. Gedroits (22) 
worked on solubility of phosphates in 2 per cent acetic and citric acids. The relative solu- 
bilities in acetic acid were tricalcium phosphate, aluminum phosphate, ferric phosphate, in 
the order named; in citric acid dicalcium phosphate and aluminum phosphate were equally 
soluble, ferric phosphate less soluble. In growing plants in sand culture with these phos- 
phates, the aluminum phosphate pots gave the highest yield, tricalcium phosphate was second, 
andiron phosphate pots a close third. Truog (75) questions the whole idea of employing 
chemical solvents as a means for determining the availability of different phosphates, basing 
his deductions on favorable results obtained with phosphates of aluminum and iron, which 
are, as a general rule, less soluble than calcium phosphate in such solvents. Elliot and Hill 
(16) had before this arrived at the same conclusions. Fraps (19), on the other hand, pro- 
posed 0.2 iV nitric acid as the solvent to indicate the available supply of phosphorus in the 
soil. He asserted that in pot experiments, the phosphoric acid removed by the crops is closely 
related to the quantity of "active" phosphoric acid. "Active" phosphoric acid is defined as 
that amount which dissolves in . 2 iV nitric acid. 

Several workers have indicated the value of dehydrating aluminum phosphate to render 
it more valuable as a fertilizer. The investigators at the Rhode Island Agricultural Experi- 
ment Station have always included roasted redondite in their comparative phosphate tests 
and have drawn attention to the value of dehydration. Morse (44) found that roasting 
increased the solubility of aluminum phosphate in neutral ammonium citrate, but pot and 
field tests failed to verify the laboratory indications of availability. Pilon et al (54) described 
a method for roasting double phosphates of iron and aluminum in order to render the com- 
bined phosphoric acid soluble in ammonium citrate. Fraps (20) pointed out that ignition 
increases the solubility of wavellite, dufrenite, and variscite inO.2 iV^ nitric acid about ten 
times and makes them almost completely soluble in 12 per cent hydrochloric acid. 
Peterson (51) conducted similar investigations and showed that heating wavellite for five hours 
at 200°C. increased the solubility of the phosphoric acid 4 to 50 per cent and heating to 
240°C. increased the solubility to 100 per cent. Dufrenite, when heated at 200°C., was but 
slightly increased in solubility. 



360 JACOBUS STEPHANTJS MARAIS 

Views concerning the comparative availability of phosphates of aluminum, iron and 

calcium 

Below we have simply an enumeration of claims and counter-claims as to the comparative 
values of aluminum, iron and calcium phosphates. Many of the statements decrying the 
value of aluminum and iron phosphates were based not on experimental work planned to test 
this particular point, but were the outcome of efforts to explain puzzling irregularities in the 
behavior of superphosphates and acid phosphates. Very many workers, too, reported on 
the topic under discussion as a side issue of a large problem and very often such work failed 
to effect a fair comparison because the individual phosphates probably display their opti- 
mum availability under unlike conditions. 

Merrill (43) reported that in most cases crude Florida rock phosphate outyielded Redonda 
phosphate. Paturel (49) advised that lime be applied to soils high in oxides of aluminum to 
prevent the fixation of phosphorus by them. Morse (44), as has already been pointed out, 
studied the solubility of aluminum phosphates and the effect of dehydration of them and 
showed that, while the solubility in neutral ammonium citrate was greatly increased, field 
tests failed to demonstrate a resulting increase in availability. Hilgard (27), as quoted in a 
former paragraph, stated that in the presence of high lime percentages, relatively low per- 
centages of phosphoric acid and potash may nevertheless prove adequate. This seems to 
indicate that Hilgard preferred calcium and magnesium as carriers of the phosphate in the 
soil to other bases. 

Deherain mentioned an experiment in France in which the action of superphosphates was 
very fleeting, due, supposedly, to the phosphoric acid passing into combination with iron 
and aluminum and so rendering the phosphate incapable of use as plant-food. Wheeler and 
Adams (83) predicted that soluble phosphates were not likely to have as good after effects on 
unlimed soil rich in iron and aluminum oxides as would bone meal and basic slag for the 
reason that the phosphoric acid would be fixed as aluminum and iron phosphates, in which 
forms plants cannot secure it readily. Gaither (21) study ng the effect of lime on the solu- 
bility of soil constituents declared that lime renders the insoluble phosphates in the soil 
soluble by replacing iron and aluminum, which are in combination with phosphorus. 

Pfeiffer and Blanck (53) analyzed the effect of alumina and silicic acid gels on the assimila- 
tion of phosphoric acid by plants and obtained results which showed that both gels reduced 
yields of plants as well as their phosphoric acid content. The experiment was conducted 
with sand fertilized with 3 gm. of basic potassium phosphate and soil extract. 

Bishop (4) worked with soybeans in pot cultures and concluded that soluble phosphates 
were not more desirable than Florida soft rock, iron and aluminum phosphates. Balentine 
(3) and later Merrill and Jordan (42), all of the Maine Agricultural Experiment Station 
working with sand cultures, found that acid phosphate gave the best returns in all cases and 
especially with the Graminae. Redondite, a phosphate or iron and aluminum, gave better 
results with Graminae than rock phosphate, but in all other cases the reverse was true. In 
the second report when these investigators worked with a larger variety of plants, they stated 
that acid phosphate was best, but the insoluble forms were utilized to a considerable extent 
and that Florida rock phosphate, on the whole, was better than iron and aluminum phos- 
phates, except for barley, corn, turnips, and potato tubers. The plants used in the investiga- 
tion were peas, clover, turnips, ruta-bagas, barley, corn, tomatoes, and potatoes. Andouard 
(1) worked with a calcareous soil and deduced that aluminum phosphate was readily available 
to plants. Burkett (7) obtained very favorable results with raw and roasted redondite. 
Gedroits (22), in pot culture with soil, declared that aluminum phosphate gave better yields 
than calcium phosphate and the latter better yields than iron phosphate. Director Pat- 
terson (50) of the Maryland Agricultural Experiment Station, made the following state- 
ment: "The iron and alumina phosphates proved in all cases to be valuable sources of phos- 
phoric acid, and it would seem that they deserve a higher rank as a fertilizer than that usually 
accorded them." 



AGRICULTURAL VALUE OF INSOLUBLE MINERAL PHOSPHATES 



361 



Nagaoka (45) employed phosphates on rice fields exhausted by continuous cropping. All 
the phosphates gave large increases in yield. Table 1 gives the relative jdelds, double super- 
phosphates being taken as 100. 

Bonomi (5), in comparing aluminum phosphate with mineral phosphate, superphosphate, 
and Thomas slag, reported that aluminum phosphate gave large increases in yield with both 
clover and wheat, but that superphosphate was always superior to it; spring wheat yields with 
aluminum phosphate was smaller than those with Thomas slag, but with clover the reverse 
was true. Elliot and Hill (16) showed that from weights of crops produced in pot experi- 
ments, iron and aluminum do not fix phosphoric acid in forms unavailable to plants; as a 
matter of fact, they claim that iron and aluminum phosphates produce more plant growth 
than the calcium compounds do. For this reason, they denounced the solvents used by 
chemists for determining the reversion of phosphates as useless for the purpose. 

TABLE 1 
Relative yields of rice as influenced by various phosphates (from Nagaoka) 



1. Double super phosphate 

2. Ferric phosphate 

3. Ferrous phosphate 

4. Aluminum phosphate . . 

5. Calcium phosphate .... 



FIRST 


SECOND 


THIRD 


FOURTH 


YEAR 


YEAR 


YEAR 


YEAR 


100 


100 


100 


100 


140 


141 


399 


58 


87 


88 


194 


44 


92 


145 


514 


103 


117 


110 


161 


118 



AVER- 
AGE 



100 
185 
103 
216 
127 



Shulov (71) worked with vivianite — a ferrous phosphate— a pure ferrous phosphate, alumi- 
num phosphates, tricalcium phosphate, and superphosphates in sand cultures. In all cases, 
the iron and aluminum phosphates proved highly efficient as fertilizer and increasing amounts 
of lime up to 1 per cent produced very little depressing effect on their action. Eaguley (2) 
compared normal orthophosphates of calcium, iron and aluminum on oats, peas, and Swedish 
turnips grown on artificial soil of sand and chalk. As a general rule, iron and aluminum 
phosphates proved more efficient than calcium phosphates. Peterson and Truog (52), in pot 
cultural work, demonstrated that freshly precipitated and dried ferric phosphate served as a 
better source of phosphorus for oats than did rock phosphate, while for rape, the results were 
exactly the reverse. Truog (75) later made the following statement: "Contrary to the 
general belief that aluminum and iron phosphates are relatively unavailable to plants, nine 
out of ten plants tested made better growth on aluminum phosphate than on calcium phos- 
phate, and six better growth on ferric phosphate." 



EXPERIMENTAL 

These experiments were planned to determine whether or not it is desir- 
able to employ mineral phosphates of aluminum and iron as sources of phos- 
phorus. Studies were made comparing their value as sources of phosphorus 
with that of calcium phosphate in various forms both natural and artifi- 
cial. Simultaneously efforts were made to determine what conditions would 
cause these phosphates to be of the greatest value for crop growth. 

Description of materials used 

The aluminum phosphates employed were lazulite from near Death Valley, 
Inyo county, California, wavellite from Cumberland county, Pennsylvania 
and Saldanha phosphate from the Cape Province in South Africa; the iron 



362 



JACOBUS STEPHANUS MARAIS 



phosphates were dufrenite from near Vesuvius, Rockbridge county, Virginia; 
and vivianite from Leadville, Colorado; the calcium phosphates were Florida 
rock and Laingsburg phosphate from the Cape Province, South Africa. Some 
of the wavellite was obtained from Montgomery county, Arkansas. Besides 
these phosphates there were also used bonemeal and acid phosphate. In 
the sand cultures disodium hydrogen phosphate in solution replaced the acid 
phosphate. Table 2 gives the analyses of the various phosphates. 

TABLE 2 
Composition of phosphates employed in experiments 

KIND OF PHOSPHATE 



Lazulite 

Wavellite 

Saldanha 

Dufrenite 

Vivianite 

Florida hard rock 

Laingsburg 

Bonemeal 

Acid phosphate . . 



PHOSPHORUS 


ALUMINUM 


IRON 


per cent 


per cent 


per cent 


13.72 


16.20 


5.04 


10.04 


17.40 


2.48 


9.14 


16.30 


1.61 


12.07 


1.60 


40.20 


9.11 


1.20 


22.40 


14.70 


4.17 


1.53 


14.01 


2.92 


2.59 


12.52 


0.00 


Trace 


7.01 


0.81 


0.40 



per cent 

0.97 

0.23 

1.06 

0.11 

0.10 

26.40 

31.90 

27.10 

14.70 



All the aluminum phosphates are basic phosphates, i.e., they have aluminum 
hydrate associated with the phosphate and all of the phosphates are more or 
less hydrated. Lazulite has the additional property of being completely 
insoluble in acids. Hot aqua regia acting on lazulite for an hour fails to dis- 
solve more than a trace of phosphoric acid. Wavellite and Saldanha phos- 

TABLE 3 

Essential plant-food elements per acre of 2,000,000 pounds of water-free soil or approximately 

the surface layer of 6% inches over one acre 



PLANT rOOD ELEMENTS 



Phosphorus 

Potassium 

Nitrogen 

Limestone requirement by Hopkins' method in pounds 
of CaCOs per acre 



BROWN SILT LOAM 


YELLOW SILT LOAM 


Ihs. 


lbs. 


1,096 


706 


32,240 


29,180 


4,287 


1,942 


400 


2,949 



phates dissolve readily in acids. Infrenite is a basic ferric phosphate con- 
taining a trace of magnesium. The formula usually ascribed to it by geologists 
is, FeP04-Fe(OH)3. Vivianite crystallizes in the monoclinic form and is a 
hydrated ferrous phosphate with the formula Fe3(P04)2 -81120. The Florida 
hard rock is rather high in aluminum as compared to the usual run of phosphate 
from this source. The Lainsburg phosphate contains quite an appreciable 
quantity of calcium carbonate. 



AGRICULTURAL VALUE OF INSOLUBLE MINERAL PHOSPHATES 363 

The pot cultures were conducted in 1-gallon glazed earthen-ware pots 
drained by a hole in the bottom of the pot and capable of holding 10 pounds 
of soil. In most of the soil cultures a light phase of brown silt loam from the 
University Farm at Urbana, lUinois, was used. As far as is known the soil 
had never been cultivated and had never received soil treatment of any kind. 
The soil is known to respond readily to applications of phosphorus. In later 
experiments a yellow silt loam soil was introduced. This soil came from near 
Vienna, in Johnson county, Illinois. 

In all soil cultures 10 pounds of soil were used per pot and in the sand cultures 
12 pounds of sand. 

Experiment 1 

This experiment was planned to test the comparative effects of the phos- 
phates on crops and the effect of lime and gyspum on their availability. 

The experiment was begun in the spring of 1920. Brown silt loam was used 
and treated as described in table 4. 

The pots were planted to buckwheat and annual white sweet clover. All 
the buckwheat pots were numbered as in table 4; the sweet clover pots were 
given the same numbers as the buckwheat pots but had "rv;" prefixed to the 
number. Each treatment was carried out in duplicate. The planting oc- 
curred on February 6, 1920. The sweet clover seed was inoculated. Twenty 
buckwheat seeds and thirty sweet clover seeds were planted in each pot. After 
the seeds were up the plants were gradually thinned so that at the end of 4 
weeks only the seven strongest buckwheat plants were left in each pot and the 
ten strongest sweet clover plants in each of the sweet clover pots. 

Much cloudy weather was experienced and this combined with the short 
days made growing conditions in the greenhouse unsatisfactory. It was 
noticed that the buckwheat especially was looking decidedly poor. In order 
to insure the elimination of all factors tending toward depression of growth it 
was thought advisable to start a new series of cultures in which the buckwheat 
would receive an application of 1.84 gm. of calcium nitrate, the equivalent of 
100 pounds of nitrogen to the acre. In all other respects the same plan of 
treatment was followed, also: 

Series 600 corresponded exactly with series 100 
Series 700 corresponded exactly with series 200 
Series 800 corresponded exactly with series 300 
Series 900 corresponded exactly with series 400 
Series 1000 corresponded exactly with series 500 

In addition, eight control pots were planted to determine the effect of 
limestone gypsum and calcium nitrate. The treatment applied to these pots 
and the yields obtained are shown in table 5. 

The planting of this series began on February 22, and was completed 
on February 24. 



364 



JACOBUS STEPHANUS MARAIS 





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AGRICULTURAL VALUE OF INSOLUBLE MINERAL PHOSPHATES 365 



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366 



JACOBUS STEPHANUS MARAIS 



From the third week in February to the end of March good growing weather 
prevailed. All the plants made good growth. On the following dates the 
plants were sprayed with nicotine sulfate to kill thrips with which they had 
become infested : March 7 and 8, April 2, 18 and 19. The spraying on March 
7 was done in cloudy weather. The weather, however, suddenly cleared up 
with the result that some of the plants were injured. " Scald" spots developed 
on the buckwheat. The 600 series suffered most. On all other occasions 
spraying was done in the evening. The crops were harvested May 1, pre- 
served in cheesecloth bags until air-dry and weighed. 

TABLE 5 
Yields of buckwheat on brown silt loam showing the effect of calcium nitrate limestone and 
gypsum applied singly and in all possible combinations 







YIELD OF CROP 




INCREASE 


NUMBER OF 


TREATMENT 








OVER 


POT 


First pot 


Second pot 


Average 


CHECK 






gm. 


gm. 


gm. 


per cent 


1101 


Calcium nitrate 


10.9 


11.7 


11.3 


28.4 


1102 


Calcium nitrate, L 


11.4 


9.9 


10.65 


22.1 


1103 


Calcium nitrate, G 


11.8 


12.0 


11.9 


35.3 


1104 


Calcium nitrate, G, L 


10.0 


10.0 


10.0 


13.7 


1105 


L 


8.9 


8.9 


8.9 


1.1 


1106 


G 


9.1 


8.8 


8.95 


1.7 


1107 


G, L 


9.4 


8.4 


8.9 


1.1 


Check 


None 


8.7 


8.9 


8.8 





DISCUSSION AND RESULTS OF EXPERIMENT 1 

The relative increase in yield over checks are significant in all cases except 
perhaps for lazulite and dufrenite. The yields and increases in yields are 
recorded in tables 4, 5, and 6. 

In table 6 the percentage increase over checks was calculated with pot 1101 
as the check. From table 5, the effect of limestone, gypsum and calcium 
nitrate may be determined. Limestone and gypsum had no apparent effect 
when applied either alone or in combination. Series 100 to 500 inclusive 
show that on buckwheat, bonemeal and acid phosphate gave the best results. 
Large increases in yield were obtained with the mineral phosphates of calcium 
on the unlimed pots and with wavellite and Saldanha on the limed pots. 
Vivianite gave substantial increases in yield in both the limed and unlimed soil. 
The yields with Florida phosphate and Laingsburg phosphate on the limed 
pots showed that the crops were benefited considerably by the addition of the 
phosphorus. On the unlimed pots small, but probably significant, increases 
in yield were obtanied where wavellite and Saldanha phosphates were used, 
Dufrenite and lazulite had little or no effect on the growth of the buckwheat. 

From series 600 to 1000, inclusive, the value of calcium nitrate when used in 
conjunction with the phosphate minerals can be determined. Table 6 shows 



AGRICULTURAL VALUE OF INSOLUBLE MINERAL PHOSPHATES 



367 



TABLE 6 
Yields of buckwheat grotvn on brown silt loam treated with various phosphates together with 

calcium nitrate 



NtJMBER OF 


TREATMENT* 


YIELD OF CROPS 


INCREASE OVER 


POT 


First pot 


Second pot 


Average 


NO. 1101 








gm. 


gm. 


gm. 


per cent 


601A 


Laz. 




14.0 


13.7 


13.85 


22.6 


602A 


Laz., 


L 


16.2 


16.1 


16.15 


42.9 


603A 


Laz., 


G 


12.8 


13.4 


13.10 


15.9 


604A 


Laz., 


G,L 


15.4 


16.2 


15.80 


39.8 


601B 


Wav. 




16.8 


16.4 


16.60 


46.9 


602B 


Wav., 


L 


20.3 


21.4 


20.85 


84.5 


603B 


Wav., 


G 


15.9 


15.7 


15.80 


39.8 


604B 


Wav., 


G, L 


19.4 


19.0 


19.20 


69.9 


601 C 


Sal. 




16.1 


16.6 


16.35 


44.7 


602C 


Sal., 


L 


19.5 


19.2 


19.35 


71.2 


603C 


Sal., 


G 


16.2 


15.3 


15.75 


39.4 


604C 


Sal., 


G,L 


19.6 


18.1 


18.85 


66.8 


701A 


Duf. 




15.4 


14.9 


15.15 


34.1 


702A 


Duf., 


L 


16.5 


15.2 


15.85 


40.3 


703A 


Duf., 


G 


15.8 


15.4 


15.60 


38.1 


704A 


Duf., 


G,L 


14.3 


14.0 


14.15 


25.2 


701B 


Viv. 




18.6 


18.5 


18.55 


64.1 


702B 


Viv., 


L 


17.4 


18.0 


17.70 


56.7 


703B 


Viv., 


G 


17.6 


17.9 


17.75 


57.1 


704B 


Viv., 


G,L 


20.2 


20.2 


20.20 


78.7 


801A 


Fl. R. 




18.0 


20.4 


19.20 


69.6 


802A 


Fl. R., 


L 


18.1 


17.9 


18.00 


59.3 


803A 


Fl. R., 


G 


19.3 


20.6 


19.95 


76.6 


804A 


Fl. R., 


G,L 


18.0 


17.7 


17.85 


57.9 


801B 


Lgg. 




20.2 


20.9 


20.55 


81.8 


802B 


Lgg. 


L 


15.6 


16.3 


15.95 


41.1 


803B 


Lgg-, 


G 


21.2 


19.9 


20.55 


81.8 


804B 


Lgg., 


G,L 


17.0 


16.6 


16.80 


48.7 


901 


Bone 




20.8 


19.7 


20.25 


79.2 


902 


Bone, 


L 


16.1 


17.1 


16.60 


46.9 


903 


Bone, 


G 


18.9 


17.9 


18.40 


62.8 


904 


Bone, 


G,L 


15.8 


16.3 


16.05 


42.0 


1001 


Ac. P. 




18.8 


20.1 


19.45 


72.1 


1002 


Ac. P. 


L 


19.0 


18.7 


18.85 


66.8 


1003 


Ac. P., 


G 


17.9 


19.6 


18.75 


65.9 


1004 


Ac. P., 


G, L 


19.4 


19.4 


19.40 


71.7 



'^These treatments were identically the same as those given in table 4. 



368 JACOBUS STEPHANUS MARAIS 

that it enhanced the value of all the phosphates except bonemeal and acid 
phosphate. Even lazulite and dufrenite in this experiment have benefited 
the buckwheat considerably. The increases in yield here show that wavellite 
and Saldanha phosphate on limed soil are on a par with the calcium phos- 
phate minerals on unlimed soil and as good as bonemeal, acid phosphate and 
vivianite. 

With sweet clover different results were obtained. Lazulite and dufrenite 
again showed no effect. Vivianite gave small but significant increases in 
yield. On unlimed soil Saldanha phosphate had no effect on crop growth 
but on the limed soil substantial increases in growth were evident. The best 
results were obtained with the calcium phosphates. Little difference could 
be discerned between these phosphates. 

It is noticeable that with the aluminum phosphate consistent gains in yield 
were made by the addition of lime; with the iron phosphate no effect was 
noticeable, and with calcium phosphates the reverse effect was to be observed. 
As already pointed out the phosphates of aluminum and iron employed in 
this experiment are basic phosphates. Aluminum phosphate according to 
Truog (77) owes its availabiUty to the relative ease with which it hydrolyses in 
neutral or nearly neutral solutions. From a chemical point of view, this 
assumption is probably correct. The salt is formed from a strong acid and a 
weak base and will, therefore, hydrolyze readily according to the following 
equation: 

/OH 
(1) AlPOi+SH^O^Alf OH4-H3PO4 
^OH 

When such a reaction takes place in the presence of plant roots, there will be 
a tendency for the phosphoric acid to be removed and the aluminum hydrate 
to remain in the soil. The net result v/ould be that the aluminum phosphate 
in the soil will become more and more basic. From the law of mass action it 
is evident that as the phosphate becomes more basic the rate of hydrolysis of 
the phosphate will diminish. As time goes on plants will experience increas- 
ing difficulty to obtain phosphorus as a result of the reaction represented in 
equation (1). The beneficial action of lime on aluminum is evident from the 
following reactions: 

(2) CaCOa + H2CO3 = CaHa (COa)^ 

(3) 2A1(0H)3 + 3CaH2(C03)2 ^ CaAWe + 6H2CO3 

The lime therefore removes the aluminum hydrate from the reaction by precipi« 
tating it as the very insoluble calcium aluminate. The continual removal of 
aluminum hydrate prevents reaction (1) from reaching an equilibrium so 
that plants will be supplied steadily with a supply of soluble phosphorus, 
The lime may, of course, precipitate the phosphoric acid as tricalcium phos- 



AGRICULTURAL VALUE OF INSOLUBLE MINERAL PHOSPHATES 369 

phate but such precipitated tricalcium phosphate has repeatedly been shown 
to be readily available. The phosphorus will become thoroughly disseminated 
in the soil and furthermore is readily rendered soluble by carbonic acid in the 
following manner: 

Ca3(P04)2 +2H2C03-^Ca2H2(P04) +CaH2(C03)2 

The fact that lime failed to increase the assimilation of ferric phosphate 
and ferrous phosphate is evident from the above explanation. Iron is a 
stronger base producing substance than aluminum as is proved by the fact that 
ferric and ferrous hydrates never behave in the capacity of acids as aluminum 
hydrate does. Lime, therefore, will have no effect upon ferric or ferrous hy- 
drate. It may be assumed that iron phosphates hydrolyze in the same manner 
as aluminum phosphates but they are not likely to hydrolyze as readily. 

(4) FePOa + 3H2O ^ Fe(OH), + H3PO4 

We would have to conclude, therefore, that as the phosphoric acid is used by 
the plants the residue will always become increasingly basic and unavailable 
to plants. Vivianite is a fairly pure ferrous phosphate, Fe3(P04)2; while 
dufrenite is a basic ferric phosphate, FeP04-Fe(OH)3. This probably ex- 
plains in part the greater availability of vivianite. Probably the utilization of 
iron phosphates by plants in the soil must be explained as being chiefly due 
to the action of acids, carbonic acid and nitrous acid, both of which are pro- 
duced in quantity in soils containing a fair amount of organic matter. 

(5) 2FeP04 + 3H2CO3 -* 2H3PO4 +2Fe2(C03) 

The Fe2(CO)3 is unstable and readily hydrolyzes to give the following: 

(6) Fe2(C03)3 + 6H20->2Fe(OH)3 + 3H2CO3 or 

(7) FePOi + 3H6 N02-^Fe(N02)3 +H 3PO4 

Chemically, one would expect that the availabihty of tricalcium phosphate 
would be suppressed by the action of limestone. Carbonic acid and nitrous 
acid produced in soil will react in part at least with the limestone. 

(8) CaCOs + H2CO3-* CaH2(C03)2 

(9) CaCOa + 2HN02-^ Ca(N02)2 + H2CO3 

Apart from this factor the introduction of the common calcium ion will tend to 
force the equilibrium of the following equation to the left rather than to the 
right. 

(10)Ca3(PO4)2+ 2H2CO3 ?± CaH2(P04)2+CaH2(C08)2 

Truog (77) was able to demonstrate by pot cultures that the introduction of 
soluble calcium ions into the soil solution tended to lower the rate of assimila- 
tion of phosphorus from tricalcium phosphate. This applies especially to 
plants which do not feed heavily on calcium. 



370 JACOBUS STEPHANUS MARAIS 

In the literature survey, Hilgard (27) was quoted as stating that in calcareous 
soils relatively smaller percentages of phosphoric acid will suffice for good 
plant growth than in acid soils. Truog (77) referring to observations of a 
decrease in growth of cereals due to the addition of lime carbonate in pot cul- 
tures, makes the following statement 

"This decrease in availability is undoubtedly due to a condition which is temporary. In 
becoming acid a soil goes into a condition which takes years to develop, and the addition of 
lime carbonate causes many profound changes, some of which may affect the availability of 
the phosphorus. The veiy favorable results obtained by investigators in long continued 
field experiments involving the use of ground limestone is strong evidence that any unfavor- 
able result at the start is due to temporary conditions." 

If we consider the fact that in a soil, and even a calcareous soil, there is con- 
siderably more aluminum, as a rule, than calcium, we cannot but believe that 
during the ages of weathering to which soils have been subjected, a considerable 
quantity of phosphorus has gone into combination with aluminum. This 
phosphorus will be readily available to plants in a calcareous medium as had 
already been explained. Is it not the aluminum phosphate in the soil rather 
than the calcium phosphate that has caused Hilgard to express the opinion 
quoted above? On the other hand, the favorable results obtained in field 
experiments as the result of long continued use of limestone may be explained 
in the following manner. Legumes in general grow better in limed soils. 
Good farm practice would, therefore, result in the incorporation of more organic 
matter in the soil and especially of more highly nitrogenous organic matter. 
The limestone creates conditions favorable for biological activity in the soil. 
The organic matter is more rapidly decomposed and hence there is rapid pro- 
duction of carbon dioxide and nitrous acid. These acids may readily produce 
acid zones in the soil. In such a heterogeneous mass as the soil, it is not difii- 
cult to conceive of acid and alkaline or neutral zones in close proximity. These 
zones will naturally not be stationary. Acid zones will continuously be formed 
and again destroyed. In the acid zones, tricalcium phosphate will be dis- 
solved and rendered available to plants; in the alkaline zones, aluminum phos- 
phates will be hydrolyzed and rendered available to crops so that, even 
there, soluble phosphorus will not be lacking entirely. Lime, as such, un- 
doubtedly reduces the availability of tricalcium phosphate but due to its 
effect on the organic matter and on the biological activities of the soil, it acts 
indirectly as a liberator of phosphorus. 

Gypsum was added to certain pots in an endeavour to stimulate root growth 
in the plants and so improve the feeding capacity of the plants. If an in- 
crease was to be expected one would have looked for it in connection with the 
use of aluminum and iron phosphates. With the calcium phosphates, the 
introduction of the common ion calcium would result in a reduction of yield 
according to Truog (77). No such reduction can be said to have been observed. 
The gypsum seems to have been without effect of any kind. It may be pointed 
out, too, at this time that the choice of calcium nitrate as a nitrogen fertilizer 



AGklCULTURAL VALUE OP INSOLUBLE MINERAL PHOSPHATES 371 

was probably unfortunate in that it may have caused a reduction in yields on 
the calcium phosphate pots, due to the introduction of the common ion cal- 
cium. On the other hand the Russian work (10) has shown that calcium 
nitrate is the best form of nitrogen to apply for buckwheat. It is decidedly 
superior to sodium nitrate. The use of ammonium salts was avoided since 
it would have introduced the factor of extremely rapid nitrification and the 
copious production of acids. 

Buckwheat and sweet clover were chosen as crops because of their reputation 
as strong feeders on insoluble phosphates. 

Experiment 2 

In order to test the various phosphates under conditions where no soil 
phosphorus was present it was thought advisable to compare their action in 
sand culture. Buckwheat and sweet clover were again chosen as the crops to 
be grown. The phosphates, lime and gypsum were applied by thoroughly 
mixing them in the sand. The rest of the required plant-food nutrients were 
added in a culture solution composed of 10 cc. of each of the following solutions 
and the mixture diluted to a liter. 

164 gm. of calcium nitrate in 2500 cc. 

50 gm. of potassium sulfate in 2500 cc. 

20 gm. of magnesium sulfate in 2500 cc. 

0.01 gm. of ferric chloride in 2500 cc. 

One liter of nutrient solution was added at the time of planting, another liter 
after 3 weeks, a third liter 2 weeks later, and thenceforth a liter was applied 
every week. The same pots were employed as in the former experiment and 
the same quantities of the phosphates, gypsum, and limestone were employed. 
Each pot contained 12 pounds of sand. The rate of application of fertilizers 
were therefore: 

Phosphates, 1000 pounds of 65 per cent rock phosphate per acre 
Gypsum, 200 pounds per acre 
Limestone, 1 ton per acre 

The fertilizers applied in the solid form were thoroughly incorporated into the 
sand. 

All the pots were planted in duplicate. The sweet clover pots had an "x" 
prefixed before each number. The buckwheat pots were planted on March 
13, 1920, and harvested on May 13. The sweet clover pots were planted on 
March 13, 1920, and harvested on May 20. Through an error, one of the 
pots in the buckwheat series did not receive any phosphorus and had to be 
discarded. 

As in the former experiment 30 seeds were planted in each pot, and as time 
went on the weaker plants were pulled out until finally the buckwheat pots 
each contained 7 plants and the sweet clover pots each 10 plants. 

On April 2, April 23, and May 4, all the pots were sprayed with nicotine 
sulfate solution to kill thrips with which the plants had become infested. 



372 



JACOBUS STEPHANUS MARAIS 



TABLE 7 
Treatments applied to various pots 



NUMBER 
OF POT 


TREATMENT IN ADDITION TO NUTRIENTS 


NUMBER 

or POT 


TREATMENT IN ADDITION TO NUTRIENTS 


I201A 


Laz.* 


1041A 


FL, R. 


1202A 


Laz., L 


1042A 


Fl. R., L 


1203A 


Laz., G 


1403A 


Fl. R., G 


1204A 


Laz., G, L 


1404A 


Fl. R., G, L 


1201B 


Wav. 


1401B 


Lgg- 


1202B 


Wav., L 


1402B 


Lgg., L 


1203B 


Wav., G 


1403B 


Lgg, G 


1204B 


Wav., G,L 


1404B 


Lgg., G, L 


1201C 


Sal. 


1501 


Ac. P. 


1202C 


Sal., L 


1502 


Ac. P., L 


1203C 


Sal., G 


1503 


Ac. P., G 


1204C 


Sal., G, L 


1504 


Ac. P., G, L 


1301 A 


Duf. 


1601 


Bone 


1302A 


Duf, L 


1602 


Bone, L 


1303A 


Duf., G 


1603 


Bone, G 


1304A 


Duf., G, L 


1604 


Bone, G, L 


1301B 


Viv. 






1302B 


Viv., L 






1303B 


Viv., G 






1304B 


Viv., G, L 







* For explanation of abbreviations see table 4. 

t Included in each liter of nutrient solution at rate of 10 cc. of solution containing 26 gm. 
Na2HP04 in 2500 cc. 



DISCUSSION AND RESULTS OF EXPERIMENT 2 

The weights of the crops produced are recorded in tables 8 and 9. 

This experiment bears out even more markedly than the former one the 
relative abihty of buckwheat and sweet clover to assimilate phosphorus from 
the various sources employed. On the unlimed pots bonemeal was on a par 
in value to disodium hydrogen phosphate in solution. Vivianite proved to be 
an excellent source of phosphorus. As in the first experiment, the addition of 
lime and gypsum had no effect on its availability. The average yield from 
the vivianite pots was equal to the average yield of all the Florida phosphate 
and Laingsburg pots. On the unlimed pots the tricalcium phosphate minerals 
proved superior to vivianite; on the limed pots, inferior. In the buckwheat 
series, wavellite and Saldanha phosphates in the limed pots were on a par with 
Florida and Laingsburg phosphates in the unlimed pots. In the sweet clover 
series this does not hold. The sweet clover made considerably better growth 
with the aluminum phosphates on the limed pots than on the unlimed pots, 



AGRICULTURAL VALUE OP INSOLUBLE MINERAL PHOSPHATES 



373 



TABLE 8 
Weights of crops of buckwheat produced in sand culture 







WEIGHT OF CROP 1 




POT NUMBER 


TREATMENT IN ADDITION TO NOT^IENTS 








YIELD 




First pot 


Second pot 


Average 








gm. 


gni. 


gm. 


per cenl 
pot 1501 = 
100 per cent 


1201A 


Laz.* 


1.20 


1.20 


1.20 


7.8 


1201B 


Wav. 


4.76 


5.70 


5.23 


34.2 


1201C 


Sal. 


5.79 


5.09 


7.44 


48.6 


1301A 


Duf. 


2.26 


2.14 


2.20 


14.4 


1301B 


Viv. 


9.77 


9.81 


9.79 


64.0 


1401A 


Fl. R. 


9.23 


9.40 


9.32 


60.9 


1401B 


Lgg- 


12.66 


10.44 


11.55 


75.6 


1501 


Ac. P. 


14.87 


15.72 


15.30 


100.0 


1601 


Bone 


15.02 


15.47 


15.25 


99.7 

per cent 
pot 1502 = 
100 per cent 


1202A 


Laz., limestone 


1.46 


1.38 


1.42 


9.5 


1202B 


Wav., limestone 


9.45 


9.98 


9.72 


65.1 


1202C 


Sal., limestone 


9.49 


8.92 


9.21 


61.7 


1302A 


Duf., limestone 


2.17 


2.26 


2.23 


14.9 


1302B 


Viv., limestone 


9.42 


10.04 


9.73 


65.2 


1402B 


Fl. R., limestone 


6.39 


6.97 


6.68 


44.7 


1402B 


Lgg., limestone 


6.00 


6.73 


6.37 


42.7 


1502 


Ac. P., limestone 


14.93 




14.93 


100.0 


1602 


Bone, limestone 


12.19 


11.49 


11.84 


79.9 

per cent 
pot 1503 = 
100 per cent 


1203 A 


Laz., gypsum 


1.17 


1.23 


1.20 


8.0 


1203B 


Wav., gypsum 


5.69 


6.31 


6.00 


40.3 


1203C 


Sal,, gypsum 


5.63 


5.40 


5.52 


36.9 


1303 A 


Duf., gypsum 


2.12 


2.28 


2.20 


14.7 


1303B 


Viv., gjrpsum 


10.01 


9.39 


9.70 


64.9 


1403A 


Fl. R., gypsum 


10.59 


11.03 


10.81 


72.3 


1403B 


Lgg-, gypsum 


10.10 


11.50 


10.80 


72.2 


1503 


Ac. P., gypsum 


14.71 


15.18 


14.95 


100.0 


1603 


Bone, gypsum 


14.37 


13.65 


14.01 


93.6 

per cent 
pot 1504 = 
100 per cent 


1204A 


Laz., limestone, gypsum 


1.39 


1.55 


1.47 


9.6 


1204B 


Wav., limestone, gypsum 


9.47 


10.16 


9.82 


64.4 


1204C 


Sal., limestone, gypsum 


10.37 


9.09 


9.73 


63.8 


1304A 


Duf., limestone, gjnpsum 


2.01 


2.41 


2.21 


14.5 


1304B 


Viv., limestone, gypsum 


10.40 


11.29 


10.85 


71.2 


1404A 


Fl. R., limestone, gypsum 


6.24 


6.73 


6.49 


42.6 


1404B 


Lgg., limestone, gypsum 


6.16 


5.80 


5.98 


39.2 


1504 


Ac. P., limestone, gypsum 


15.81 


14.66 


15.24 


100.0 


1604 


Bone., limestone, gypsum 


11.60 


9.18 


10.39 


68.2 



■ For explanation of abbreviations see table 4. 



374 



JACOBUS STEPHANUS MARAIS 



TABLE 9 
Weights of crops of sweet clover produced in sand cultures 







WEIGHT or CROPS 




POT NUMBER 


TREATMENT IN ADDITION TO NUTRIENTS 








YIELD 




First pot 


Second pot 


Average 








gm. 


gm. 


gm. 


per cent 
pot xlSOl = 
100 per cent 


X1201A 


Laz.* 


2.18 


2.29 


2.24 


19.7 


X1201B 


Wav. 


2.51 


2.61 


2.56 


22.5 


X1201C 


Sal. 


2.6 


2.86 


2.73 


24.0 


X1301A 


Duf. 


2.40 


2.11 


2.31 


20.3 


X1301B 


Viv. 


8.53 


9.82 


9.18 


80.8 


X1401A 


Fl.R. 


10.37 


10.54 


10.46 


92.1 


X1401B 


Lgg- 


10.13 


10.06 


10.15 


89.3 


xl501 


Ac. P. 


11.74 


10.98 


11.36 


100.0 


xl601 


Bone 


12.53 


13.02 


12.78 


112.5 

per cent 
pot xl 502 = 
100 per cent 


X1202A 


Laz. limestone 


3.01 


3.10 


3.56 


26.6 


X1202B 


Wav., limestone 


6.72 


6.50 


6.61 


49.4 


X1202C 


Sal., limestone 


7.35 


7.38 


7.37 


55.0 


X1302A 


Duf., limestone 


2.26 


2.09 


2.18 


16.3 


X1302B 


Viv., limestone 


9.20 


8.41 


8.81 


65.8 


X1402A 


FI. R., limestone 


8.63 


8.40 


8.52 


63.6 


X1402B 


Lgg., limestone 


7.31 


7.74 


7.53 


56.2 


xl502 


Ac. P., limestone 


13.42 


13.36 


13.39 


100.0 


xl602 


Bone, limestone 


10.19 


10.42 


10.31 


76.9 

per cent 
potxl503 = 
100 per cent 


X1203A 


Laz., gypsum 


2.00 


1.92 


1.96 


15.6 


X1203B 


Wav., gypsum 


3.14 


2.89 


3.02 


24.1 


X1203C 


Sal., gypsum 


2.67 


2.81 


2.74 


21.9 


X1303A 


Duf., gypsum 


2.37 


2.17 


2.27 


18.1 


X1303B 


Viv., gypsum 


10.54 


9.18 


9.86 


78.6 


X1403A 


Fl. R., gypsum 


12.04 


11.09 


11.57 


92.3 


X1403B 


Lgg., gypsum 


9.68 


9.71 


9.70 


77.4 


xl502 


Ac. P., gypsum 


12.61 


12.47 


12.54 


100.0 


xl602 


Bone, gypsum 


13.35 


12.72 


13.04 


103.9 

per cent 
potxl504 => 
100 percent 


X1204A 


Laz., limestone, gypsum 


2.98 


2.89 


2.93 


21.8 


X1204B 


Wav., limestone, gypsum 


6.71 


7.22 


6.97 


51.9 


X1204C 


Sal., limestone, gypsum 


8.13 


8.17 


8.15 


60.7 


X1304A 


Duf., limestone, gypsum 


2.42 


2.27 


2.35 


17.5 


X1304B 


Viv., limestone, gypsum 


9.79 


9.81 


9.80 


73.0 


X1404A 


Fl.R., limestone, gypsum 


8.86 


8.28 


8.57 


63.9 


X1404B 


Lgg., limestone, gypsum 


6.80 


7.45 


7.13 


53.1 


xl504 


Ac. P. limestone, gypsum 


13.29 


13.54 


13.42 


100.0 


X1604 


Bone, limestone, gypsum 


10.01 


10.76 


10.39 


77.4 



For explanation of abbreviations see Table 4. 



AGRICULTURAL VALUE OF INSOLUBLE MINERAL PHOSPHATES 375 

but the yields were not as large as those obtained on the unlimed pots fer- 
tilized with the tricalcium phosphate minerals. The Florida and Laingsburg 
phosphates, as with the buckwheat, proved inferior in the limed pots to the 
same phosphates in the unlimed pots, but in this case equally as good as wavel- 
lite and Saldanha on the limed pots. Lazulite and dufrenite behaved as they 
did in the first experiment, proving themselves poor sources of phosphorus. 

A fact to be recorded and probably of some significance is that in the earlier 
stages of growth of the buckwheat the big differences in total growth on the 
aluminum phosphate were not so much in evidence. It was during the last 
4 or 5 weeks of growth that the plants on the limed pots displayed a greater 
rate of growth than those on the unlimed pots. Plate 1, figure 1, shows the 
buckwheat at the age of 6 weeks. The effects of liming is plainly evident 
where the calcium minerals were applied but not nearly so well marked where 
aluminum minerals were used. Figure 2 shows that where sweet clover was 
grown liming showed very marked influence from the very beginning. The 
pot marked xl705 was one of a series that was discarded because of the series 
becoming infected with red spider. This pot received a complete nutrient 
solution in which the phosphorus was supplied in the form of monocalcium 
phosphate. Besides this the pot was limed and treated with 14 gm. of Florida 
rock phosphate, i.e., rock phosphate at the rate of 7 tons per acre. 

These observations tally with the explanation as to assimilability of the 
phosphates of aluminum and calcium, i.e., in an unlimed medium the availa- 
bility of aluminum phosphate will decrease as time goes on, whereas the effect 
of lime on the calcium phosphates will be in evidence immediately. This is 
considered strong evidence in favor of the explanation as to the effect of lime 
on the availability of aluminum phosphate. 

It is remarkable that similar results have been obtained with buckwheat 
and sweet clover. Both crops, of coarse, are known to be heavy feeders on 
phosphates; but, on the one hand, buckwheat has a rather limited rooting sys- 
tem while sweet clover, on the other hand, has a very extensive rooting system. 
It seems, therefore, that the two plants should vary considerably in feeding 
power or else in the manner in which they feed. It is possible to conceive of the 
idea that the sweet clover may have been injured by aluminum on the alu- 
minum phosphate series. Soluble aluminum in any form would be injurious 
to sweet clover. This perhaps explains why sweet clover did not respond as 
well as buckwheat to treatment with aluminum phosphate. Sweet clover 
roots probably excrete more carbonic acid than do buckwheat roots. Alu- 
minum phosphate is not as readily dissolved by carbonic acid as is tricalcium 
phosphate. Buckwheat may feed more heavily on phosphorus rendered 
soluble by hydrolysis by reason of a more rapid removal of phosphorus from 
the root-hairs to the growing parts of the plant. It must be borne in mind 
that in these sand cultures, microorganisms do not play the part that they 
do in soils. None of the pots were inoculated with soil infusion and no 
nitrifiable material was added. In all probability all pots were infected with 



376 JACOBUS STEPHANUS MARAIS 

some kind or kinds of organisms but it is not likely that any organisms that 
could affect the availability of phosphorus appreciably could have been 
present, or even if they had been present could have exerted any influence, so 
that the plants had to obtain phosphorus by one of the following methods: 

1 . The solution of phosphates in the nutrient medium. 

2. By hydrolysis and consequent solution of phosphates. 

3. By solvent effect of acid root excretions, which would be, according to Czapek (13), 
chiefly through the agency of carbonic acid. 

Phosphorus brought into solution by the first two methods should be equally 
available to both crops. The phosphorus obtained by the third method would 
depend on the individuality of the crop in regard to the quantity of the car- 
bonic acid excreted. The differences in feeding power between the two crops 
under the conditions of the experiment would in all probability have to be 
ascribed to the rate of carbonic acid excretion, unless there is a difference 
between the plants in the rate at which phosphorus is translocated from the 
root hairs to the growing parts of the plants. 

It was thought that in sand culture we would be able to duplicate Truog's 
(77) results with regard to the effect of soluble calcium salts on the availa- 
bility of the tricalcium phosphate minerals (i.e., reduce it); but gypsum, as in 
soil cultures, appeared to have no effect on the growth of the plants. It is, 
of course, possible that due to the large excess of calcium already present in 
the form of calcium nitrate the additional effect of the calcium ions from the 
gypsum was too smaU to register an appreciable difference in crop growth. 

Experiment 3 

The purpose of this experiment was to determine the effect of nitrification 
of urea on the availability of the various phosphates both in soil and sand 
cultures. 

The experiment was commenced in the fall of 1920. The media for growth 
employed were the brown silt loam and the yellow silt loam described in the 
first section of this paper and pure quartz sand. Throughout the experiment 
phosphorus and limestone were applied in the same quantities as in former 
experiments. In the case of the sand cultures the nutrient was applied at the 
same rate as in previous sand cultures. Where the pots received urea, 0.75 
gm. was applied to each pot, i.e., such a quantity was added that, if all the 
nitrogen in it were converted to nitric acid, enough acid would be formed to 
displace exactly all the phosphoric acid from the tricalcium phosphate ap- 
plied to each of the pots treated with it. The pots, which did not receive urea, 
received an application of 2.05 gm. of calcium nitrate, i.e., nitrogen in equal 
quantity to that added to the pots receiving urea. 

The treatments were applied to all the pots and water added to the optimum 
amount in each pot. The sand pots each received 50 cc. of a soil infusion. 
The pots were then left unplanted for 14 days, the object being to allow the 
urea to be nitrified before the germination of the seed. 



AGRICULTURAL VALUE OF INSOLUBLE MINERAL PHOSPHATES 377 



TABLE 10 
Treatments applied to individual pots 



TREATMENT 


BROWN SILT 
LOAM SERIES 
POT NtJMBERS 


YELLOW SILT 
LOAM SERIES 
POT NUMBERS 


SAND CULTURE 

SERIES 
POT NUMBERS 


Laz., Ca(N03)2* 
Laz., Ca(N03)2, L 
Laz., urea 
Laz., urea, L 


llA 
12A 
13A 

14A 


61A 
62A 
63A 
64A 


lllA 
112A 
113A 
114A 


Wav., Ca(N03)2 
Wav., Ca(N03)2, L 
Wav., urea 
Wav., urea, L 


IIB 
12B 
13B 
14B 


61B 
62B 
63B 
64B 


lllB 
112B 
113B 
114B 


Sal., Ca(N03)2 
Sal., Ca(N03)2, L 
Sal., urea 
Sal., urea, L 


lie 

12C 
13C 
14C 


61C 
62C 
63C 
64C 


lllC 
112C 
113C 
114C 


Duf., Ca(N03)2 
Duf., Ca(N03)2, L 
Duf., urea 
Duf., urea, L 


21 
22 
23 
24 


71 

72 
73 
74 


121 
122 
123 
124 


Fl. R., Ca(N03)2 
Fl. R., Ca(N03)2, L 
Fl. R., urea 
Fl. R., urea, L 


31A 
32A 
33A 
34A 


81A 
82A 
83A 
84A 


131A 
132A 
133A 
134A 


Lgg., Ca(N03)2 
Lgg., Ca(N03)2, L 
Lgg., urea 
Lgg., urea, L 


31B 
32B 
33B 
34B 


81B 
82B 
83B 
84B 


131B 
132B 
133B 
134B 


Bone, Ca(N03)2 


41 


91 


141 


Bone, Ca(N03)2, L 


42 


92 


142 
143 
144 


Bone, urea 
Bone, urea, L 


43 
44 


93 
94 


Ac. P., Ca(N03)2 
Ac. P., Ca(N03)2, L 
Ac. P., urea 
Ac. P., urea, L 


51 

52 
53 

54 


101 
102 
103 
104 


151 
152 
153 
154 


Ca(N03)2 
Ca(N03)2, L 
Urea 
Urea, L 


Check 1 
" 2 
" 3 
" 4 


Check 1 
" 2 
" 3 
« 4 


No check 
group 



■ For explanation of abbreviations see table 4. 



378 JACOBUS STEPHANUS MARAIS 

The materials added to the pots were ground together in a mortar and 
thoroughly mixed before they were incorporated in the growing media. Thor- 
ough mixing of the media and the material was effected. 

Five series of pots were used, two containing brown silt loam, one contain- 
ing yellow silt loam, and two containing sand. For each soil, one series was 
planted to wheat. The two extra series were planted to annual white sweet 
clover and each of these pots was numbered the same as its corresponding 
wheat pot except for an "x" prefixed to the number. The pots in the sand 
series each received in addition a nutrient solution containing magnesium sul- 
fate, potassium sulfate, and ferric chloride. The concentrations of the salts in 
this solution, the manner and time of application are exactly as described 
under experiment 1. The treatments applied to the individual pots in all the 
series are indicated in table 10. 

Sweet clover pots in the first series were planted on October 25 and 26, 
1920; the wheat pots, on October 27 and 28, 1920; and a second series of 
pots containing sand were planted to annual white sweet clover on November 
7, 1920. 

DISCUSSION AND RESULTS OF EXPERIMENT 3 

Due to poor light conditions during the winter months all the plants made 
very slow growth so that harvesting occurred only toward the middle of March. 
During the growing season the greenhouses were fumigated on two occasions 
with "nicofume" to rid the plants of aphis. On three separate occasions 
spraying with nicotine sulfate was resorted to in order to kill the thrips with 
which the plants had become infested. The last 6 weeks of the growing periods 
when the days were becoming longer the plants grew most rapidly. The sweet 
clover, especially, remained stunted to a considerable extent in the earlier grow- 
ing period. 

On March 4, the wheat on the sand cultures was harvested. On March 
13 and 14, the wheat on the soil cultures was harvested and on March 18, all 
the sweet clover pots were harvested. The crops were kept in paper bags 
dried in an oven at 105°C. and weighed. 

The weights of crops obtained are recorded in tables 11, 12, 13, 14, and 15. 

From the above tables the percentage increase in growth as a result of the 
treatments can be determined. In general, the yields from the pots not treated 
with urea substantiate the findings of the first experiment with respect to the 
comparative availability of the various phosphates and the effect of lime on 
the assimilabihty of the phosphorus. With wheat, acid phosphate gave easily 
the best results on both limed and unlimed soil, while wavellite and Saldanha 
phosphates on limed soil and bonemeal on unlimed soil were second in their 
effect. Florida rock and Laingsburg phosphates were effective on the un- 
limed soil but failed to produce much increased growth on the limed pots. 
The sweet clover yields on the brown silt loam series showed that, even where 
pots had been limed, the tricalcium phosphates were a better source of phos- 



AGRICULTURAL VALUE OF INSOLUBLE MINERAL PHOSPHATES 



379 



TABLE 11 
Weight of wheat from brown silt loam series 







YIELD OF CROP 


INCREASE 




POT NUMBER 


TREATMENT 








OVER 








First pot 


Second pot 


Average 


CHECK 1 








Sw. 


gm. 


gm. 


per cent 




llA 




5.63 


5.73 


5.68 


-1.6 




IIB 


Wav., Ca(N03)2 


6.68 


6.20 


6.53 


13.2 




lie 


Sal., Ca(N03)2 


7.57 


8.42 


7.99 


38.3 




21 


Duf., Ca(N03)2 


5.67 


5.58 


5.63 


-2.4 




31A 


Fl. R., Ca(N03)2 


8.37 


8.92 


8.65 


49.9 




31B 


Lgg., Ca(N03)2 


8.69 


8.31 


8.50 


47.3 




41 


Bone, Ca(N03)2 


9.57 


9.07 


9.32 


61.5 




51 


Ac. P., Ca(N03)2 


10.72 


10.31 


10.52 


82.3 




Check 1 


Ca(N03)2 alone 


5.73 


5.81 


5.72 








INCREASE 














OVER CHECK 2 




per cent 


12A 


Laz., Ca(N03)2, L 


6.32 


6.41 


6.37 


10.4 


13.6 


12B 


Wav. Ca(N03)2,L 


8.64 


9.42 


9.03 


56.5 


61.5 


12C 


Sal , Ca(N03)2, L 


8.57 


9.42 


9.00 


56.3 


61.0 


22 


Duf., Ca(N0s)2, L 


5.71 


5.46 


5.59 


-3.1 





32A 


Fl. R., Ca(N03)2, L 


7.09 


7.17 


7.13 


23.6 


27.5 


32B 


Lgg., Ca(N03)2,L 


6.23 


6.26 


6.25 


8.3 


11.8 


42 


Yone., Ca(N03)2, L 


8.18 


7.84 


8.01 


38.8 


43:3 


52 


Ac. P., Ca(N03)2, L 


10.03 


10.67 


10.35 


79.4 


85.2 


Check 2 


Ca(N03)2, L 


5.71 


5.46 


5.59 


-3.1 






INCREASE 














OVER CHECK 3 




per cent 


13A 


Laz., urea 


12.80 


13.27 


13.04 


125.9 


25.0 


13B 


Wav., urea 


14.95 


14.15 


14.55 


152.2 


39.5 


13C 


Sal., urea 


14.71 


15.88 


15.30 


165.1 


46.7 


23 


Duf., urea 


13.69 


15.31 


14.50 


153.2 


39.0 


33A 


Fl. R., urea 


20.82 


19.70 


20.26 


251.1 


94.2 


33B 


Lgg., urea 


19.68 


19.97 


19.83 


243.7 


90.1 


43 


Bone, urea 


18.50 


19.82 


19.16 


232.1 


83.7 


53 


Ac. P, urea 


16.41 


16.84 


16.63 


188.2 


59.5 


Check 3 


Urea alone 


10.09 


10.77 


10.43 


80.7 






INCREASE 














OVER CHECK 4 




per cent 


14A 


Laz., urea, L 


10.73 


10.75 


10.74 


86.3 


23.6 


14B 


Wav., urea, L 


10.96 


12.00 


11.48 


98.9 


31.9 


14C 


Sal., urea, L 


11.74 


12.54 


12.14 


110.4 


39.7 


24 


Duf., urea, L 


12.07 


11.42 


11.80 


104.5 


35.7 


24A 


Fl. R., urea, L 


17.76 


16.55 


17.16 


197.4 


97.5 


34B 


Lgg., urea,L 


14.80 


14.03 


14.42 


149.9 


65.9 


44 


Bone, urea, L 


17.25 


16.80 


17.03 


195.1 


95.9 


54 


Ac. P., urea, L 


15.35 


15.24 


15.30 


165.2 


76.1 


Check 4 


Urea,L 


8.48 


8.89 


8.69 


50.6 





380 



JACOBUS STEPHANUS MARAIS 



TABLE 12 
Yields of sweet clover from brown silt loam soil series 







YIELD OF CROA 


INCREASE 




POT NUMBER 


TREATMENT 








OVER 
X CHECK 1 






First pot 


Second pot 


Average 








gm. 


gm. 


gm. 


per cent 




xllA 


Laz., Ca(N03)2 


4.V8 


4.41 


4.60 


-3.4 




xllB 


Wav., CaCNOs)! 


5.44 


5.70 


5.57 


17.0 




xllC 


Sal., Ca(N03)2 


4.65 


5.48 


5.06 


6.3 




x21 


Duf., Ca(N03)2 


4.70 


5.47 


5.09 


6.9 




x31A 


Fl. R., Ca(N03)2 


7.93 


7.15 


7.54 


58.4 




x31B 


Lgg., Ca(N03)2 


7.46 


7.26 


7.36 


54.6 




x41 


Bone, Ca(N03)y 


10.57 


9.66 


10.12 


112.6 




x51 


Ac. P., Ca(N03)2 


6.72 


7.90 


7.31 


53.6 




xCheck 1 


Ca(N03)2, alone 


4.80 


4.22 


4.76 








INCREASE 














OVER 














X CHECK 2 




per cent 


xl2A 


Laz., Ca(N03)2, L 


5.45 


4.95 


5.20 


9.5 


-0.4 


xl2B 


Wav., Ca(N03)2,L 


6.00 


6.46 


6.23 


30.9 


19.3 


xl2C 


Sal., Ca(N03)2, L 


6.84 


6.13 


6.49 


36.3 


24.3 


x22 


Duf., Ca(N03)2,L 


5.44 


5.31 


5.38 


13.0 


3.1 


x32A 


Fl. R., Ca(N03)2, L 


6.68 


7.05 


6.87 


44.3 


31.6 


x32B 


Lgg., Ca(N03)2,L 


6.70 


6.23 


6.47 


35.9 


23.9 


x42 


Bone, Ca(N03)2, L 


8.70 


7.80 


8.25 


73.3 


58.0 


x52 


Ac. P., Ca(N03)2, L 


10.69 


9.56 


10.13 


112.8 


94.1 


xCheck 2 


Ca(N03)2, L 


5.12 


5.32 


5.22 


9.7 






INCREASE 














OVER 














X CHECK 3 




per cent 


xl3A 


Laz., urea 


4.87 


4.53 


4.70 


-1.3 


13.3 


xl3B 


"Wav., urea 


5.62 


6.21 


5.92 


24.4 


42.7 


xl3C 


Sal., urea 


5.11 


5.24 


5.18 


8.8 


27.2 


x23 


Duf., urea 


7.21 


6.80 


7.01 


47.3 


68.9 


x33A 


Fl. R,, urea 


8.12 


8.37 


8.25 


73.3 


98.8 


x33B 


Lgg., urea 


6.24 


7.67 


6.96 


46.2 


67.7 


x43 


Bone, urea 


7.33 


8.03 


7.68 


61.3 


85.1 


x53 


Ac. P., urea 


7.55 


7.23 


7.39 


55.3 


33.3 


xCheck 3 


Urea alone 


4.38 


3.92 


4.15 


-12.8 






INCREASE 














OVER 














X CHECK 4 




per cent 


xl4A 


Laz., urea, L 


6.72 


5.91 


6.41 


34.7 


19.1 


xl4B 


Wav., urea, L 


7.34 


6.30 


6.82 


43.3 


26.8 


xl4C 


Sal., urea, L 


6.73 


6.92 


6.83 


43.3 


26.8 


x24 


Duf., urea, L 


6.24 


6.42 


6.33 


32.9 


17.7 


x34A 


Fl. R., urea, L 


9.64 


8.99 


9.33 


96.0 


73.4 


x34B 


Lgg., urea, L 


9.67 


8.94 


9.31 


95.6 


73.1 


x44 


Bone, urea, L 


9.90 


10.86 


9.38 


97.1 


74.4 


x54 


Ac. P., urea, L 


10.89 


11.74 


11.32 


137.8 


110.4 


xCheck 4 


»JlCa, Li. 


5.29 


5.47 


5.38 


13.0 





AGRICULTURAL VALUE OF INSOLUBLE MINERAL PHOSPHATES 



381 



phorus for this plant than aluminum and iron phosphates. The effect of liming 
was concordant with the former findings; it was in the relative powers of assim- 
ilating the various phosphates that the plants differed. Wheat uses alumi- 
num phosphates more readily than does sweet clover. Sweet clover is a heavy 



TABLE 13 

Yields of wheat from pots in yellow silt loam series. {A large number of crops were lost due 

to the oven, in which they were being dried, becoming overheated and crops being charred) 







YIELD OF CROPS 


INCREASE 




POT NUMBER 


TREATMENT 








OVER 
CHECK 1 






First pot 


Second pot 


Average 








gm. 


gm. 


gm. 


per cent 




61A 


Laz., Ca(N03)2 


5.36 


4.87 


5.12 


-1.2 




61B 


Wav., Ca(N03)2 


7.48 


7.11 


7.30 


40.8 




61C 


Sal., CaCNOs)? 


5.87 


5.63 


5.75 


11.0 




81B 


Lgg., Ca(N03)2 


7.23 


7.11 


7.17 


38.4 




Check 1 


Ca(N03)2, alone 


5.18 




5.18 








INCREASE 














OVER CHECK 2 




per cent 


62A 


Laz., Ca(N03)2,L 


6.12 


5.85 


5.98 


15.4 


2.9 


62B 


Wav., Ca(N03)2, L 


7.37 


9.05 


8.21 


58.5 


41.3 


62C 


Sal., Ca(N03)2, L 


8.75 


8.44 


8.60 


66.0 


48.0 


82B 


Lgg., Ca(N03)2,L 


6.40 


5.90 


6.15 


18.7 


5.9 


Check 2 


Ca(N03)2, L 


5.81 




5.81 


12.2 






INCREASE 














OVER CHECK 3 




per cent 


63A 


Laz., urea 


6.14 


6.29 


6.22 


20.1 


0.2 


63B 


Wav., urea 


6.35 


6.53 


6.44 


24.3 


3.7 


63C 


Sal., urea 


7.89 


10.22 


9.06 


74.9 


45.9 


83B 


Lgg., urea 


9.10 


10.07 


9.59 


85.1 


54.4 


Check 3 


Urea alone 


6.21 




6.21 


20.0 






INCREASE 














OVER CHECK 4 




per cent 


64A 


Laz., urea, L 


8.26 


7.71 


7.99 


54.2 


30.1 


64B 


Wav., urea, L 


9.69 


8.44 


9.07 


75.1 


47.5 


64C 


Sal., urea, L 


10.07 


10.35 


10.21 


97.1 


49.8 


84B 


Lgg., urea, L 


8.42 


7.97 


8.20 


58.4 


33.5 


Check 4 


Urea, L 


5.92 


6.31 


6.12 


18.1 





feeder on calcium, wheat a light feeder. It is to be expected, therefore, that 
in the presence of lime or even in the absence of lime sweet clover would be 
capable of assimilating the phosphorus of tricalcium phosphate more readily 
than would wheat. Truog (77) is of the opinion that oats feed heavily on 
the natural phosphates of the soil because of their large fibrous root system. 



382 



JACOBUS STEPHANUS MARAIS 



Wheat, which is very similar to oats in its general structure and manner of 
feeding, should therefore also feed heavily on soil phosphorus. The results of 

TABLE 14 
Yields of wheat from pots in the sand series 







YIELD OF CROP 


POT NUMBER 


TREATMENT IN ADDITION TO NUTRIENT SOLUTION 










First pot 


Second pot 


Average 






gm. 


gm. 


gm. 


lllA 


Laz., Ca(N03)2 


1.90 


1.28 


1.59 


lllB 


Wav., Ca(N03J2 


3.66 


4.05 


3.86 


lllC 


Sal., Ca(N03)2 


3.16 


2.22 


2.69 


121 


Duf., Ca(N03)2 


1.72 


2.04 


1.88 


131A 


Fl. R., Ca(N03)2 


2.00 


1.95 


1.98 


131B 


Lgg., Ca(N03)2 


2.18 


2.07 


2.13 


141 


Bone, Ca(N03)2 


3.11 


2.26 


2.69 


151 


Ac. P., Ca(N03)2 


10.04 


20.21 


19.63 


112A 


Laz., Ca(N03)2, L 


1.53 


1.31 


1.42 


112B 


Wav., Ca(N03)2,L 


5.40 


5.01 


5.21 


112C 


Sal., Ca(N03)2, L 


4.06 


4.16 


4.11 


122 


Duf., Ca(N03)2,L 


2.23 


2.04 


2.14 


132A 


Fl. R., Ca(N03)2, L 


1.65 


1.54 


1.60 


132B 


Lgg., Ca(N03)2, L 


2.30 


2.12 


2.21 


142 


Bone, Ca(N05)2, L 


2.38 


2.16 


2.27 


152 


Ac. P., Ca(N03)2, L 


17.62 


19.40 


18.51 


113A 


Laz., urea 


1.60 


2.10 


1.85 


113B 


Wav., urea 


4.89 


5.21 


5.05 


113C 


Sal., urea 


6.01 


6.00 


6.01 


123 


Duf., urea 


2.46 


2.67 


2.57 


133A 


Fl. R., urea 


4.30 


5.44 


4.87 


133B 


Lgg., urea 


4.19 


3.90 


4.05 


143 


Bone, urea 


7.58 




7.58* 


153 


Ac. P., urea 


19.34 


20.02 


19.68 


114A 


Laz., urea, L 


1.90 


1.25 


1.58 


114B 


Wav., urea, L 


4.77 


5.02 


4.90 


114C 


Sal., urea, L 


6.20 


6.25 


6.23 


124 


Duf., urea, L 


1.64 


2.00 


1.82 


134A 


Fl. R., urea, L 




2.22 


2.22* 


134B 


Lgg., urea, L 


2.40 


2.30 


2.35 


144 


Bone, urea, L 


2.68 




2.68* 


154 


Ac. P., urea, L 


19.59 


19.97 


19.78 



* Weights from only one pot available. The duplicate plants had died soon after ger- 
mination, presumably from the toxic effect of either the urea or ammonia formed from it. 

the above experiments justify his conclusions, for the wheat grown in sand 
culture made but poor growth. On the other hand, the wheat has responded 
very markedly to phosphate treatment on the soils. It seems logical to be- 



AGRICULTURAL VALUE OF INSOLUBLE MINERAL PHOSPHATES 



383 



lieve that the more rapid development of a root system in the soil due to the 
presence of some readily available phosphorus accounts for the greater ability 

TABLE 15 
Yields of sweet clover from the sand series 









YIELD OF CROP 




POT NUMBER 


TREATMENT IN ADDITION TO NUTRIENT SOLUTION 










First pot 


Second pot 


Average 






gm. 


gm. 


gm. 


xlllA 


Laz., Ca(N03)2 


0.84 


0.53 


0.69 


xlllB 


Wav., Ca(N03)2 


4.00 


5.20 


4.60 


xlllC 


Sal., Ca(N03)2 


5.26 


5.53 


5.40 


xl21 


Duf., Ca(N03)2 


1.33 


1.03 


1.18 


xl31A 


Fl. R., Ca(N03)2 


7.86 


10.23 


9.05 


xl31B 


Lgg., Ca(N03)2 


9.90 


9.73 


9.82 


xl41 


Bone, Ca(N03)2 


10.27 


9.97 


10.12 


xl51 


Ac. P., Ca(N03)2 


11.46 


10.17 


10.82 


xll2A 


Laz., Ca(N03)2, L 


1.19 


1.14 


1.17 


xll2B 


Wav., Ca(N03)2, L 


6.22 


6.10 


6.16 


X112C 


Sal, Ca(N03)2, L 


6.20 


7.93 


7.07 


xl22 


Duf., Ca(N03)2, L 


1.00 


1.03 


1.02 


xl32A 


F1.R., Ca(N03)2,L 


8.23 


9.35 


8.79 


xl32B 


Lgg., Ca(N03)2, L 


7.73 


7.05 


7.39 


xl42 


Bone, Ca(N03)2, L 


7.20 


7.04 


7.12 


xl52 


Ac. P., Ca(N03)2,L 


11.57 


10.84 


11.21 


xll3A 


Laz., urea 


0.48* 


0.12* 




xll3B 


Wav., urea 


0.56* 


0.77* 




xll3C 


Sal., urea 


* 


* 




xl23 


Duf., urea 


* 


* 




xl33A 


Fl. R., urea 


7.62 


* 




xl33B 


Lgg., urea 


8.45 


9.39 


8.92 


xl43 


Bone, urea 


10.49 


>*> 


10.49t 


xl53 


Ac. P., urea 


7.42 


4.32* 


7.42t 


xll4A 


Laz., urea, L 


1.12 


1.43 


1.28 


xll4B 


Wav., urea, L 


3.24 


3.54 


3.39 


X114C 


Sal., urea, L 


4.84 


5.46 


5.15 


xl24 


Duf., urea, L 


* 


1.79 


1.79t 


xl34A 


Fl. R., urea, L 


* 


* 




xl34B 


Lgg., urea, L 


9.27 


8.04 


8.66 


xl44 


Bone, urea, L 


8.38 


* 


8.38t 


xl54 


Ac. P., urea, L 


11.70 


10.92 


11.31 



* Part or all of plants died within first 3 weeks, 
t From one pot only. 

to use phosphorus applied to soil. The poor growth in sand cultures, where 
only insoluble mineral phosphates are present, is due to the inability to de- 
velop a root system in the early growth stages. Sweet clover, with its high 



384 JACOBUS STEPHANUS MARAIS 

feeding capacity for calcium, finds enough phosphorus in early stages of growth 
to develop an extensive root system; and, therefore, in later stages is better 
equipped to forage for its phosphorus. Sweet clover, of course, always has a 
very much more extensive root system than wheat. The comparison would 
be more plain if we could compare buckwheat and wheat. It must be borne 
in mind, however, that the application of calcium nitrate may have reduced 
the availability of phosphorus for wheat more than for clover, which is capable 
of utilizing more calcium. It would be interesting to grow wheat in sand cul- 
ture with insoluble phosphates and supplying it with a very small quantity 
of soluble phosphorus just after the germination of the plants. 

THE ACTION OF TIREA 

Some remarkable results have been obtained as a result of tlie action of 
urea. The object of the addition of urea was to determine the effect of ni- 
trification on the availability of the phosphates. It was an ideal source of 
organic matter to use because of its being free from phosphorus and conver- 
tible only into nitrous (or nitric) and carbonic acids, thus leaving no mineral 
residue in the soil. Furthermore, urea is nitrified very rapidly. The urea is con- 
verted into ammonium carbonate by the urea organisms present in most soils. 
The ammonium carbonate is rapidly transformed into nitrous acid and carbonic 
acid. On the brown silt loam where urea was used without phosphate treat- 
ment very curious results were obtained. With wheat, very large increases 
in growth were evident, more in the unlimed pots than in the limed ones. 
With sweet clover, urea had practically no effect, while on the yellow silt 
loam series the effect of urea was small. It is evident that in the brown silt 
loam the acid production as a result of nitrification resulted in the liberation 
of a considerable quantity of plant food. The reduction in yield of wheat 
where lime too was applied lends strength to this statement. The failure of 
response by sweet clover is the result of toxic effect on the plants by the acids 
produced. The lesser effect of urea on wheat in yellow silt loam series is due to 
the poor quahty of the soil and the inability of the soil to nitrify the urea as 
rapidly as the brown silt loam did. There is even a possibility that the urea 
remained unchanged in the soil long enough to injure the young wheat seed- 
lings. No such injury was visible at any time, however. 

THE EPEECT OF THE UREA ON THE AVAILABILITY OF THE PHOSPHATES 

The effect of the urea on the availabihty of the phosphates themselves 
was very remarkable. In many instances yields were almost trebled. Wheat 
benefited considerably more than sweet clover. The increases in yield due 
to various treatments are recorded in tables 11, 12, and 13. It is evident that 
urea exerted its greatest influence on the tricalcium phosphates. Where 
sweet clover was grown, no, or only small, increases were observed with alu- 
minum phosphates. On the yellow silt loam series, lazulite was the only 



AGRICULTURAL VALUE OF INSOLUBLE MINERAL PHOSPHATES 385 

aluminum phosphate which benefited by the presence of the urea. An out- 
standing feature is the remarkable benefit derived by sweet clover from urea 
and dufrenite in the unlimed series, especially as dufrenite has proved to be 
a poor source of phosphorus thus far. The last column on the right of tables 
11, 12, and 13 give the increases in yield that may be ascribed as due to the 
phosphorus alone. It is very doubtful whether the figures can be said to repre- 
sent the influence of the phosphates because the factors regulating growth are 
much too intricate to be separated in such an arbitrary fashion; but at the 
same time they show the percentage increase of growth as due to combined 
efforts of the urea and phosphates as against the influence of the urea or the 
calcium nitrate. 

Plate 2 shows photographs of the various wheat pots which give a clear 
picture of the effect of the various treatments. The tremendous increase 
in growth can only be ascribed to the ability of the urea to render plant 
food available as a result of its transformation into acids. The small quantity 
of soil in each pot and hence the relatively small quantity of available plant- 
food in each pot combined with the large number of plants growing in each pot 
made it difficult for the plants to grow to any considerable size without the 
addition of fertilizing materials. The roots made a tangled mass in the soil 
penetrating into every nook and corner of the pots. Hopkins and Whiting 
(30) showed that nitrite bacteria could dissolve seven times as much phos- 
phorus from rock phosphate as would be required by a growing plant in a 
medium where this phosphate was the only supply of a base. The urea was 
in intimate contact with the phosphorus and, therefore, admirably situated 
for the acids produced to act on the phosphates, rather than on other soil 
materials. All the pots were well stocked with nitrogen. It is evident then 
that in the pots not treated with urea the plants suffered from phosphate 
starvation, while in the pots treated with urea, phosphorus was dissolved in 
plenty and the extensive network of roots in the pots ensured the utilization 
of a large proportion of the phosphorus thus placed at the disposal of the plants. 
An observation which lends support to this is the fact that the wheat, growing 
on the urea-treated pots, developed heads and seed, while only in isolated 
cases did any of the heads develop at all on any of the other pots. Seed pro- 
duction and early maturity are coupled with good phosphorus supply. The 
depression in yields where lime was used together with urea and phosphate 
and especially tricalcium phosphates, must be explained as due to the neutrali- 
zation by the lime of part of the acids formed. 

The smaller response to urea by aluminum phosphate is probably due to 
the fact that aluminum is a questionable base for the nitrifying organisms 
which are known to respond to calcium and magnesium as bases. 

In the sweet clover series the formation of soluble aluminum salts was in 
all probability detrimental to the plants to which they are known to be very 
toxic. The difference in yield between check 3 and check 4 in table 12 is large 
enough to conclude that urea alone had injured the plants but that in the pres- 



386 



JACOBUS STEPHANUS MARAIS 



ence of lime it was beneficial. The results from the sand series bear out the 
above statements. Urea without lime benefited the wheat by supplying at 
least some soluble phosphorus; on the limed pots the solvent action of the acids 
was removed so that the calcium phosphates showed hardly any benefit from 
the addition of urea, but with the aluminum phosphates the lime rendered the 
aluminum phosphates available so that but small differences were noticeable. 
Wheat is not so susceptible to the toxic action of aluminum salts of acidity. 
In the sweet clover series urea and phosphate without lime caused havoc 
amongst the plants. All the pots treated with aluminum phosphate had their 
plants severely injured or killed. Even some pots in limed series suffered. 
This points strongly in the direction that sweet clover was injured by soluble 
aluminum salts or by acids, or by ammonium or ammonium nitrite. 



NITROGEN AND PHOSPHORUS CONTENT WITH AND WITHOUT UREA 

The analyses of some of the crops grown in this experiment (table 16) in- 
dicate that the differences in growth are due to the supply of available phos- 

TABLE 16 
Analyses of wheat from sand series 



POT NUMBER 


TREATMENT 


NITROGEN 


PHOSPHORUS 






per cent 


per cent 


lllC 


Sal., Ca(N03)2 


2.37 


0.048 


112C 


Sal., Ca(N03)2,L 


2.03 


0.054 


113C 


Sal., urea 


2.42 


0.076 


114C 


Sal., urea, L 


2.23 


0.056 


131B 


Lgg., Ca(N03), 


1.96 


0.046 


132B 


Lgg., Ca(N03)2,L 


2.04 


0.047 


133B 


Lgg., urea 


2.12 


0.057 


134B 


Lgg., urea, L 


1.95 


0.047 


141 


Bone, Ca(N03)2 


2.01 


0.048 


142 


Bone, Ca(N03)2, L 


2.02 


0.050 


143 


Bone, urea 


2.03 


0.059 


144 


Bone, urea, L 


2.03 


0.046 



phorus rather than to any other causes. The analyses were of wheat grown in 
the sand series — both nitrogen and phosphorus content were determined. 

Experiment 4 

The purpose of this experiment was to determine the availability of chemi- 
cally pure phosphates of aluminum, iron, and calcium and the effect of igni- 
tion on the availability of mineral phosphates of calcium, iron, and aluminum. 

In the first part of the experiment, the chemically pure phosphates were 
compared, with and without the effect of lime, and in order to insure that the 
calcium phosphate was not placed at a disadvantage by the use of calcium 
nitrate, comparative pots were planted in which ammonium sulfate was used 



AGRICULTt'EAL VALUE OF INSOLUBLE MLN'ERAL PHOSPHATES 



387 



as a source of nitrogen. With the aluminum and iron phosphates a small 
quantity of calcium silicate was added as a source of calcium where ammonium 
sulfate was used. 

TABLE 17 
Yields of buckwheat on pots with chemically pure phosphates and ignited mineral phosphates 









WEIGHT OF CROP 


POT >njiIBER 


TREATMENT IN ADDITION TO NTTRIEXT SOLUTION 










First pot 


Second pot 


Average 








ivi. 


gm. 


gm 


1 


Al. P., 


(NH4)2S04,CaSi03 


3.00 


3.30 


3.15 


2 


Al. P., 


(NH4)2S04, L,CaSi03. 


4.67 


4.64 


4.66 


3 


Al. P., 


Ca(N03)2 


6.20 


6.63 


6.42 


4 


Al. P., 


Ca(X03)2, L 


6.92 


7.00 


6.96 


11 


Fe. P., 


(NH4)2S04,CaSi03 


4.47 


4.55 


4.51 


12 


Fe. P., 


(XH4)2S04, L,CaSi03 


4.71 


4.35 


4.53 


13 


Fe. P., 


Ca(N03)2 


5.39 


6.20 


5.80 


14 


Fe. P., 


Ca(N03)2, L 


6.27 


5.67 


5.97 


21 


Ca. P., 


(XH4)2S04 


4.31 


4.25 


4.28 


22 


Ca. P., 


(NH4)2S04, L 


3.16 


3.18 


3.17 


23 


Ca. P., 


Ca(N03)2 


6.65 


6.92 


6.79 


24 


Ca. P., 


Ca(X03)2, L 


4.51 


4.83 


4.67 


31 


Laz. I., 


Ca(N03)2 


3.33 


3.65 


3.49 


32 


Laz. I., 


Ca(N03)2, L 


4.76 


4.70 


4.73 


2>2> 


Laz., 


Ca(N03)2 


1.56 


1.40 


1.48 


34 


Laz., 


Ca(N03)2, L 


2.14 


2.57 


2.36 


41 


Wav.L, 


Ca(N03), 


5.96 


5.87 


5.92 


42 


Wav. I., 


Ca(N03)2, L 


7.49 


8.05 


7.77 


43 


Wav., 


Ca(N03)2 


2.80 


2.47 


2.64 


44 


Wav., 


Ca(N03)2, L 


3.59 


3.18 


3.39 


51 


Sal. I., 


Ca(N03)2 


5.93 


6.06 


6.00 


52 


Sal. I., 


Ca(X03)2, L 


7.89 


7.87 


7.88 


53 


Sal., 


Ca(X03)2 


3.03 


3.50 


3.27 


54 


Sal., 


Ca(X03)2, L 


4.95 


4.04 


4.50 


61 


Duf. I., 


Ca(X03)i 


2.24 


2.29 


2.27 


62 


Duf.I., 


Ca(N03)2 L 


2.00 


2.29 


2.15 


63 


Duf., 


Ca(X03)2 


1.40 


1.86 


1.63 


64 


Duf., 


Ca(N03)2, L 


1.60 


1.76 


1.68 


71 


Fl. R. I. 


, Ca(X03)2 


6.31 


5.96 


6.14 


72 


Fl. R. I. 


, Ca(X03)2, L 


4.80 


4.56 


4.68 


73 


Fl. R., 


Ca(X03)j 


6.11 


6.21 


6.16 


74 


Fl. R., 


Ca(X03)2, L 


4.48 


4.77 


4.63 



* Key to Abbreviations: Al. P., Aluminum phosphate; Fe. P., pure ferric phosphate; 1., 
ignited. 

The medium used was pure quartz sand which received the same nutrient 
solution at the same intervals as described in experiment 3. The limestone 
and other salts were applied in the same manner and quantities as in the 
former experiments. 



SOIL SCIENCE, VOL. XIII, NO. 5 



388 JACOBUS STEPHANUS MARAIS 

These pots were planted on January 17, 1920, and harvested March 29. 
On three separate occasions the plants were dusted with tobacco dust to con- 
trol the thrips with which they had become infested. The buckwheat plants 
were thinned until 6 remained in each pot. 

The crops, when harvested, were placed in paper bags, dried in an oven 
at 105°C., and weighed. Table 17 gives the weights of crops obtained. 

DISCUSSION AND RESULTS OF EXPERIMENT 4 

The weights of crops obtained are well in accord with what one might expect 
from the results of experiment 1. Calcium nitrate is evidently a far better 
form of nitrogen for buckwheat than is ammonium sulfate. Even with calcium 
phosphate, calcium nitrate proved to be the better form of nitrogen in spite 
of the fact that ammonium sulfate is supposed to enhance the assimilability 
of tricalcium phosphates. It is fortunate that calcium nitrate was chosen for 
the latter half of the experiment as a source of nitrogen. Of the pure phos- 
phates, we may safely state aluminum phosphate is as available as calcium 
phosphate and that these two are only slightly superior to ferric phosphate as 
a source of phosphorus for buckwheat. The effect of lime on the availa- 
bility of the pure aluminum phosphate was not so much in evidence as it was 
with the mineral phosphates of aluminum. This is to be expected. The 
aluminum phosphate is free to hydrolyze readily at the beginning, having no 
aluminum hydrate associated with it. Only as time goes on and the aluminum 
hydrate begins to accumulate does the lime exert its influence on the availa- 
bility of the phosphates. With iron and calcium phosphates the effect of 
lime resulted as expected. The slight effect of lime on pure aluminum phos- 
phate, the failure to affect ferric phosphate at all and the great depression in 
availability of pure tricalcium phosphate is cited again as strong evidence in 
support of the theory described under the first experiment as to the effect of 
lime on the availability of aluminum, iron, and calcium phosphates. 

In connection with the relative availabilities of the three types of phosphates 
to crops, it must be borne in mind that a plant with great ability to utilize 
tricalcium phosphate was employed, as has been demonstrated time and again. 
It is very probable that had wheat, oats, millet, flax, or some such low-feeding- 
power plant been used instead of buckwheat, aluminum and iron phosphates 
would have shown up to better advantage since their phosphorus is rendered 
available more readily by hydrolysis than by the action of roots through the 
agency of carbonic acid. Acids have less solvent action on aluminum and 
iron phosphates than on calcium phosphates as has been demonstrated in 
experiment 3. 

Ignition of the phosphates has had a remarkable effect on the assimil- 
ability of the phosphorus. It may be stated here that the phosphates were 
ignited at a bright red heat for 5 hours. Saldanha phosphate and wavellite 
lost about 15 per cent of their weight; the other three phosphates about 5 



AGRICULTURAL VALUE OF INSOLUBLE MINERAL PHOSPHATES 389 

per cent. This loss of weight was, of course, taken into account when apply- 
ing the phosphates so that all phosphates were applied on an equal phosphorus 
basis. Ignition of the three aluminum phosphate minerals resulted in doub- 
ling the crops obtained with them as compared with the unignited minerals. 
Smaller but still significant increases in yields were realized by igniting dufren- 
ite. Ignition has had no influence on the availability of the calcium phosphate 
used. 

The increased availability of the aluminiun and iron phosphates may be due 
to two causes: 

1. Dehydration of the mineral and dehydration of the aluminum and ferric 
hydrates associated with the phosphate. 

2. Destruction of the physical structure of the minerals. The first cause 
seems to be the more logical one. As has been explained in experiment 1, 
the basic phosphates are less available than the non-basic ones. With the 
lazulite, wavellite, and Saldanha phosphates we have associated a quantity 
of aluminum hydrate. Ignition converts the hydrates into oxides: 

2Al(OH)3^Al203 + 3H2O 

With dufrenite we have associated ferric hydrate which is dehydrated: 

2Fe(OH)3-»Fe202 + 3H2O 

Now aluminum and ferric oxides are far less soluble than aluminum and ferric 
hydroxides, so that they would, therefore, exert a less depressing effect on the 
availability of the phosphates. No doubt these oxides will slowly again be 
converted into hydrates in the soil, but this hydration is slow and furthermore 
in the case of aluminum hydrate, as hydration proceeds, the lime will precipi- 
tate the aluminum as calcium aluminate. 

2A1(0H)3 + 3Ca(OH)2->Ca3Al206 + 6H2O 

This precipitation will also be more effective than when the lime has to act on 
the aluminum hydrate en masse. The inability of lime to remove the ferric 
hydrate explains the smaller effect of ignition on the availability of the 
dufrenite. 

In the unlimed pots treated with ignited aluminum phosphate and in all the 
pots treated with ferric phosphate, the process of hydration of the oxides is 
gradual so that most of the alumina and ferric oxide will at first occur as 
partially hydrated oxides: AIO(OH), Al20(OH)4, FeO(OH), Fe20(OH)4, and 
numerous others. These partially hydrated oxides are not as soluble as the 
fully hydrated ones and would therefore exert less influence on the solubility 
of the phosphates, their hydrolysis, and final assimilation by the plants. 

The destruction of physical structure of the minerals may, of course, be an 
important consideration. The solubility of the minerals may readily be 
greatly altered by destruction of the crystalline structure. The natural 



390 



JACOBUS STEPHANUS MARAIS 



solubility of the minerals will greatly affect the rate of hydrolysis of the various 

minerals. There are large possibilities for accounting for many riddles with 

regard to phosphates on the basis of crystalline structure of the minerals. 

Quartaroli (63) claims the existence of two dicalcic phosphates which he 

Ca 
represents schematically by Ca/Ca/(HP04)2, and „ 

is amorphous and transformable into Ca(H2P04)2 and the second is crystalline 
and not transformable into Ca(H2P04)2. He suggests the possibility of four 
forms of tricalcium phosphates which he represents schematically as: 

U) / Ca / Ca / Ca / (P04)2 



(HP04)2. The first 



(B) 



(C) 



(D) 



Ca 


Ca 




Ca 


Ca 


Ca 




Ca 



(P04)5 



(P04)5 



Ca 
Ca 
Ca 



(P04)5 



(A) would be gradually transfonnable into dicalcium phosphate, then mono- 
calcium and finally into phosphoric acid; (B) would be transformable into 
the crystalline type of dicalcium phosphate and would not be able to produce 
any monocalcium phosphate; (C) would pass from tricalcium phosphate to 
the monocalcium phosphate without yielding the dicalcium phosphate and 
finally form phosphoric acid; (D) cannot be converted into di- or monocalcium 
phosphate but passes directly to phosphoric acid. Quartaroli has proven the 
presence of two lithium phosphates. He claims that phosphorites are mixtures 
of the four forms of calcium phosphate. Aluminum and iron phosphates would 
lend themselves to the production of similar isomers. Perhaps the variability 
inter se in availability of calcium phosphates, aluminum phosphates, and iron 
phosphates is due to the varying proportions of the different isomers in the 
several minerals. Ignition may or may not alter the proportions of the various 
isomers and so exert its effect on the availability of the various phosphates. 
The differences in the availability of lazulite, wavellite, and Saldanha phos- 
phates, that of Florida rock and Laingsburg phosphate, etc., may easily 
be due to the proportions of the various possible isomers as suggested by 
Quartaroli. 

It is very remarkable that ignited wavellite and Saldanha phosphates on 
the limed pots should have produced larger crops than even any of the pure 
phosphates. The availability of lazulite was much increased as a result of the 
ignition but in no form did the lazulite prove nearly as good as wavellite or 
Saldanha phosphates. This proves that there is a fundamental difference 
between the aluminum phosphates in these three minerals. Neither the 
crystalline structure nor the presence of aluminum hydrate can be designated 



AGRICULTURAL VALUE OF LNSOLUBLE MLNERAL PHOSPHATES 391 

as the reason. It seems that Quartaroli has made a very notable contribution 
to our understanding of the phosphates we deal with in agriculture. 

Plate 3, figure 2, shows the relative growth made by the buckwheat with 
the ignited and imignited aluminum phosphate minerals. 

Experiment 5 

In this experiment an attempt was made to illustrate some factors, which 
affect the availability of phosphates. 

The first factor studied was the solubility of alumimmi phosphate in an 
alkaline solution. The plan followed was similar to that of Kossovitsch (37) 
described on page 356 in the survey of literature. Three pots were planted. 
Inside each pot there was placed a porous pot made of bauxite. The porous 
pot had the same depth as the gallon pots used throughout this experiment 
and had a diameter of about 4 inches. This pot allowed the penetration of 
crystalloids in solution but effectively withstood root penetration through 
its walls. 

In pot 1, both the inner and the outer pots were filled with sand. The outer 
pot received an application of lime and ferric chloride. The rest of the plant- 
food materials were applied in a nutrient solution containing monocalcium 
phosphate, potassium sulfate, and magnesium sulfate. 

In pot 2, the inner pot received an application of 10 gm. of wavellite well 
mixed with the sand and the outer pot, an application of lime and ferric 
chloride. The rest of the plant nutrients were added in the form of a nutrient 
solution containing potassium sulfate, magnesium sulfate, and potassium 
carbonate. 

The nutrients were all applied in the manner and amounts already described, 
except that one-third of the potassium sulfate was replaced by an equivalent 
in potassium of potassiima carbonate. 

In pot 3, was a dupHcate of pot 2, except that the wavellite was applied in 
the outer pot instead of the inner pot. 

Inoculated annual white sweet clover seed was sowed in the outer pots. All 
the pots were planted in duplicate. The nutrient solutions were applied only 
through the inner pot. All the water added was applied to the inner pot, 
so that the soil solutions reaching the plant roots all passed through the walls 
of the porous pot. Planting occurred on March 23, 1920, harvesting on May 
27, 1920. 

During the first month of the growing period, it was not thought that the 
plants in pot 2 would survive. On pot 1, the sweet clover grew luxuriantly. 
On pot 3, fairly good growth was obtained. At the end of the month the 
plants in pot 2 suddenly began growing, those near the porous pot first, those 
farther away in succession until all the plants were growing. Ten plants were 
finally left in each pot. The yields are given in table 18. 

Enough growth was obtained on pot 2 to give confidence that the plants 
obtained phosphorus. Thus, phosphorus must have been dissolved by the 



392 



JACOBUS STEPHANUS MARAIS 



nutrient solution and diffused through the walls of the porous pots. In 
alkaline soils, then, aluminum phosphate will be dissolved by the soil solution. 
This fact is in accord with Storer's statement (73). Plate 4, figure 1, 
shows a photograph of the roots of the plants clustered around the porous 
pots. Chemotaxis is probably the cause for the location of the roots of the 
plants. 

The second factor studied was the solvent effect of plant roots on various 
phosphates. 

Thin, flat, smooth-surfaced plates of plaster of Paris were made. While 
the plaster was setting, phosphate was dusted onto one surface through a 200- 
mesh sieve and smoothed over the surface, so that the entire surface was cov- 
ered with a thin uniform layer of phosphate which was also firm and smooth. 
All portions of the plates not covered with phosphate were carefully painted 
with "asphaltum" paint. With a little practice very satisfactory smooth 
phosphate surfaces may be produced. Such plates were made using Saldanha 

TABLE 18 
Yields of sweet clover 







WEIGHT OF CROPS 




POT NUMBER 










First pot 


Second pot 


Average 




gm. 


gm. 


gm. 


1 


14.25 


13.62 


13.94 


2 


3.27 


2.67 


2.97 


3 


7.87 


8.7 


8.31 



phosphate, wavellite and Laingsburg phosphate. Each plate was placed 
vertically in a pot of sand moistened with a nutrient solution containing po- 
tassium sulfate, magnesium sulfate, and ferric chloride in the proportions 
already described in former experiments. The sand also received an appli- 
cation of limestone and gypsum, the latter at the rate of 200 pounds per 
acre. There were two plates of each phosphate. In the one case, a sweet 
clover plant about 6 inches tall was placed with its roots against the phos- 
phate surface, in the other the plant was set on the side opposite to which the 
phosphate was to be found. Eight weeks after planting, the plants were 
removed from the pots and the roots and the plates examined. Plates 4 
and 5, accurately depict the effect of the plant roots on the phosphate sur- 
faces. The roots of the plants were matted all over the surface of the phos- 
phate, a large portion of which had been removed. The pitted appearance 
of the plates marked clearly the corrosive effect of the roots. 

The sweet clover plants using calcium phosphate developed slightly better 
than the other four plants but the use of only one plant precludes the drawing 
of any conclusions as to the best phosphate in this form. 



AGRICULTURAL VALUE OF INSOLUBLE MINERAL PHOSPHATES 393 

The plant roots could find phosphate only in very limited area. Nearly 
all the roots were confined to that area. The demonstration, while giving no 
actual proof, indicates that root contact with the insoluble phosphates is im- 
portant as a factor in the assimilation of such phosphorxis and that roots of 
sweet clover ha,ve considerable ability in rendering phosphates soluble, prob- 
ably as a result of acid excretion. It was noticeable that the roots growing 
against the phosphate plates were considerably flattened. 

GENERAL DISCUSSION 

The phosphorus of the soil occurs in the form of organic compounds and 
minerals; the latter chiefly as aluminum, calcium, iron, and magnesium phos- 
phates. These mineral phosphates may of course be in the form of complexes 
with organic matter as suggested by Peterson (51), especially if the phosphates 
are basic ones. Acid humic bodies and acid silicates may readily form com- 
pounds with basic phosphates. Truog (77) points out that these compounds 
may be "very resistant and insoluble compounds." 

It is the problem of agriculturists to furnish growing crops a plentiful supply 
of phosphorus to be drawn from the stock in the soil. Each of the various 
phosphorus-containing compounds has a different degree of availability and, 
what is perhaps more important, demands certain special conditions for its 
maximum availability which vary with each type of phosphate. The ideal 
practice for the farmer is to obtain such conditions as will yield him the largest 
quantity of available phosphorus per acre. In order to prevent the deteriora- 
tion of the land, this involves maintaining and generally, too, increasing the 
stock of phosphorus in his soil. The necessity of growing and plowing under 
legumes to add nitrogen to the soil involves almost invariably the use of lime- 
stone. Few legumes thrive in acid soils. Apart from the question of growing 
legumes it is a known fact that the organisms involved in transforming or- 
ganic nitrogen into the nitrate form thrive best on calcareous media. Cen- 
turies of profitable employment of lime is proof enough of the value and the 
necessity of its use. Drainage, cultivation, liming, and incorporation of or- 
ganic matter with the soil are essential farm practices on most of the arable 
soils of the world. These practices must be followed. The kind and amount 
of phosphorus compounds to be applied, the time and manner of application, 
the kind of crops grown, the use of catch crops, the employment of other 
fertilizers not involving phosphorus are the factors which the farmer may use 
in order to make the best use of the phosphorus of the soil and of insuring a 
good supply of phosphorus to the crops he grows. 

The investigations reported in this paper have shown that plants can utilize 
aluminum, iron, and calcium phosphates to some extent. Certain forms of 
each of these phosphates are better than others; certain conditions improve, 
other conditions impair,' the availabiHty of these phosphates. Under all 
conditions, however, plants are able to obtain some phosphorus from any of 
the minerals used. The greater the stock of phosphorus in the soil, then the 



394 JACOBUS STEPHANUS MARAIS 

greater the amount the plants can obtain. The greater amount of surface 
exposed to agencies tending to dissolve the phosphorus, and the greater amount 
of contact of phosphate with the plant roots are two favorable factors which 
would multiply the effectiveness of the extra phosphate. 

Insuring the presence of a large quantity of phosphorus in the soil is the 
solution of the fundamental soil problem. Hopkins (29) claims that good 
farming practice renders 1 per cent of the phosphorus in the surface layer 
available every year. Twenty-three hundred pounds of phosphorus per acre 
would insure the availability of sufficient phosphorus to produce maximum 
crops of such plants as corn, oats, alfalfa, wheat, etc. If Hopkins' claim is 
true, the first step in the solution of the phosphorus problem of the soil would 
be to raise the phosphate stock of the soil to the above amount. 

The choice of the type of phosphorus to add is the second problem. The 
experiments reported above would indicate that as an average, calcium phos- 
phates are to be preferred. In a soil well stocked with limestone, however, 
aluminum phosphates may be more desirable. There is no doubt that very 
satisfactory results may be obtained by the use of aluminum phosphate. 
The price of the material would be the big factor in determining the choice 
of phosphates. Aluminum phosphates should never be used on acid soils 
unless, of course, lime is applied at the same time. In fact it would be prefer- 
able to apply lime together with the aluminum phosphate so that the two may be 
in intimate contact in the soil. Tricalcium phosphates are used with greater 
effect on acid soils. Iron phosphates have a doubtful value. If the phosphate 
is not basic it may be applied to advantage but the lasting effects will be 
much lower than that for calcium phosphates or aluminum phosphates on 
limed soils. In choosing phosphates to apply to soil, discretion should be 
used. No phosphate material should be used without a preliminary test. 
The low assimilability of the phosphorus in dufrenite and lazulite is a warning 
against indiscriminate buying of these phosphates. 

Aluminum, iron, and calcium phosphates vary as to the manner in which 
they are rendered available in the soil. It would perhaps be a good policy to 
apply both aluminum and calcium phosphates to the soil so as to make full use 
of all the reactions which tend to place phosphorus at the disposal of plants. 

Considering the time of applying phosphorus, it would be wise to apply phos- 
phorus for the green-manuring crop especially if clover, sweet clover, rape, 
mustard, or some such heavy feeder on phosphorus is used. These crops will 
then place the phosphorus they have used at the disposal of the money crops 
following. This practice would be especially desirable where aluminum phos- 
phates are used With calcium phosphates, it would perhaps be more de- 
sirable to plow the phosphate into the soil with the green-manuring crop in 
order to utilize to the fullest extent the acids produced during nitrification 
of the nitrogenous material and at the same time placing the phosphorus in 
intimate contact with the big source of carbonic acid production. The urea 
experiment reported above is further support of the results of Hopkins and 



AGRICULTUIIAL VALUE OF INSOLUBLE MINERAL PHOSPHATES 395 

Whiting (30) regarding the efifect of nitrification on the availability of 
phosphates. 

Truog (77) makes a suggestion for a rotation of crops in Wisconsin in which 
he introduces white mustard and rape as catch crops, the former being planted 
after wheat harvest, the latter, the following year in corn at the last cultivation. 
The third year clover is seeded in the oats, the fourth year the field remains in 
clover. This suggestion is an admirable one in the direction of keeping the 
soil well supplied with organic matter and in using other crops to help the weak 
feeding Graminae to obtain readily available phosphorus. 

Acid phosphate and soluble phosphates, in general, are usually too expensive 
to have a place in building up the phosphorus stock of a soil. They can be 
used with effect in another direction. If small top dressings of this phosphate 
be used, they will serve to give the young seedlings a rapid start so that they 
wUl develop a strong root sj^stem which will then function in feeding the plant 
in later growth stages. This practice should be used only in connection with 
a system in which adequate provision is made for stocking the soil with phos- 
phorus. If not, the practice will prove to be one of the best ways of rendering 
a poor soil poorer. 

CONCLUSIONS 

1. Mineral phosphates of aluminum and iron are valuable sources of phos- 
phorus for plants; under certain conditions they are superior to calcium phos- 
phate, under others inferior. 

2. Nitrification of urea with the consequent production of acids acts very 
favorably in assisting plants to obtain phosphates of almninum, iron, and cal- 
cium for food. 

3. Chemically pure phosphates of aluminum and iron are as readily avail- 
able to the plants tested as is pure calcium phosphate. 

4. Mineral phosphates of aluminum and iron are not as readily available 
as the pure phosphates of the same metals due to the fact that most of them are 
hydrated basic phosphates. 

5. Igniting the minerals, thereby dehydrating the bases associated with the 
phosphates and destroying the crystalline structures of the minerals, removes 
the drawback against the use of mineral phosphates of aluminum and iron. 

6. Aluminum phosphates, whether chemically pure or in mineral form, 
ignited or unignited, always display their maximum effect in a calcareous 
medium. 

7. The effect of iron phosphates is neither enhanced nor depressed by the 
addition of limestone under the conditions of the experiment. 

8. Under the conditions of the experiments, where chiefly neutral growing 
media were used, tricalcium phosphates were affected adversely by the addi- 
tion of limestone. 

9. An alkaline soil solution dissolves aluminum phosphate and aids the plant 
in obtaining its phosphorus for food. 



396 JACOBUS STEPHANUS MARAIS 

10. Contact of the roots of plants with mineral phosphates is a very im- 
portant factor in the assimilation of the phosphorus by plants for food. 



(1 

(2 
(3 

(4: 

(5 

(6: 

(7 
(8 

(9: 
(lo: 

(11 

(12 
(13 

(14: 

(15 

(16: 

(17 
(18 

(19: 
(2o: 

(21 



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AGRICULTURAL VALUE OF INSOLUBLE MINERAL PHOSPHATES 397 

Gedroits, K. K. 1903 Chemical methods for the determination of the fertility of 

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HiLGARD, E. W. 1914 Soils, p. 365. MacMillan, New York. 
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Hopkins, C. G., and Whiting, A. L. 1916 Soil bacteria and phosphates. 111. Agr. 

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KossoviTSCH, P. S. 1901 Ammonium salts as a source of nitrogen for plants. In 

Zhur. Opuitn. Agron. (Russ. Jour. Exp. Landw.), v. 2, no. 5, p. 625-638 
KossoviTSCH, P. S. 1902 The role of the plant in dissolving the plant food of the soil. 

In Zhur. Opuitn. Agron. (Russ. Jour. Exp. Landw.), v. 3, no. 2, p. 145-180. 
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V. 5, p. 482-493; abs. in Exp. Sta. Rec, v. 16, p. 1048. 
KossoviTSCH, P. S. 1909 Plant, phosphorite, and soil according to experiments in 

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Merrill, L. H., and Jordan, W. H. 1895 Investigations on the foraging powers of 

some agricultural plants for phosphoric acid. In Me. Agr, Exp. Sta. Rpt., 1895, 

p. 10-18. 



398 JACOBUS STEPHANUS MARAIS 

(43) Merrill, L. H. 1898 Box experiments with phosphoric acid from diEferent courses. 

In Me. Agr. Exp. Sta. Ann. Rpt., 1898, p. 64-74. 

(44) Morse, F. W. 1903 The effect of moisture on the availability of dehydrated phos- 

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(45) Nagaoka, M. 1904 On the action of various phosphates upon rice plants. In 

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Sta. Rec, v. 14, p. 127. 

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Wis. Agr. Exp. Sta. Res. Bui. 19. 

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Chem. Zentbl., v. 2, no. 22, p. 1706. 



AGRICULTURAL VALUE OF INSOLUBLE MINERAL PHOSPHATES 399 

(62) Prianishnikov, D. N., et al 1912 Experiments with different phosphates. In 

Izv. Moscov Selsk. Khov. Inst. (Ann. Inst. Agron. Moscou), v. 18, no. 1, 
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Inst. (Ann. Inst. Agron. Moscou), no. 1, p. 32-73; abs. in Exp. Sta. Rec, v. 22, 
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Khoz. Inst. (Ann. Inst. Agron. Moscou), v. 17, no. 2, p. 177-198. Abs. in Exp. 
Sta. Rec, v. 26, p. 427-428. 

(72) Soderbaum, H. G. 1915 Certain factors which influence the fertilizing action of 

slightly soluble phosphates. In K. Landtbr. Akad. Handl. och Tidskr., v. 54, 
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(73) Storer, F. H. 1887 Agriculture, v. 2, p. 169. Schribners, New York. 

(74) Truog, E. 1912 Factors influencing the availability of rock phosphate. Wis. Agr. 

Exp. Sta. Res. Bui. 20. 

(75) Truog, E. 1914 Availability of phosphate to various crops. Wis. Agr. Exp. Sta. 

Bui. 240, p. 23. 

(76) Truog, E. 1915 A new theory regarding the feeding power of plants. In Science, 

n. s., v. 41, no. 1060, p. 616-618. 

(77) Truog, E. 1916 The utilization of phosphates by agricultural crops, including a 

new theory regarding the feeding power of plants. Wis. Agr. Exp. Sta. Res, 
Bui. 41. 

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p. 169-195. 

(79) Ulbricht, W, 1892 Ueber die dugende Wirkung des Redonda, Alto Veto und Los 

Roques Phosphats. In Landbote, 1892, no. 11, p. 79. 

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v. 19, p. 102-103. 

(81) Warington, R. 1900 The comparative value of nitrate of sodium and sulphate of 

ammonium as manures. In Jour. Roy. Agr. Soc, v. 61; p. 300-346. 

(82) Wheeler, H. J., Adams, G. E. 1906 A comparison of nme different phosphates 

upon Umed and unlimed land with several varieties of plants. R. I. Agr. Exp. 
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(83) Wheeler, H. J., Adams, G. E. 1907 Continued test of nine phosphates with different 

plants. R, LAgr. Exp. Sta. Bui. 118. 



400 



JACOBUS STEPHANUS MARAIS 



(84) Wheeler, H. J. 1910 After effects of certain phosphates on limed and unlimed land. 

In Jour. Indus. Engin. Chem., v. 2, no. 4, p. 133-135. 

(85) ZeCCHINI, M. 1900 A new phosphatic manure; prepared phosphate of alumina. 

In Chem. Industriale, Jan. 1900; ahs. in Chem. News, v. 81, p. 210-212. 



PLATE 1 
Fig. 1. Buckwheat six weeks old in sand culture. 



Pot number Treatment 

1201 A Lazulite 

1202A Lazulite and lime 

1203 A Lazulite and gypsum 

1204A Lazulite, lime and gypsum 

1301 A Dufrenite 

1302 A Dufrenite and lime 

1303A Dufrenite and gypsum 

1304A Dufrenite, lime and gypsum 



Pot number Treatment 

1201C Saldanha 

1202C Saldanha and lime 

1203C Saldanha and gypsum 

1204C Saldanha, gypsum and lime 

1201B WaveUite 

1202B Wavellite and lime 

1203B Wavellite and gypsum 

1204B Wavellite, gypsum and lime 



Fig. 2. Sweet clover six weeks old 

Pot number Treatment 

xl705 Mono calcium phosphate 

xl202C Saldanah phosphate and 

lime 
xl201C. Saldanah phosphate 

alone 
xl202B Wavellite and lime 

xl201B Wavellite alone 

xl302B Vivianite and lime 



in sand cultures showing effect of lime 

Pot number Treatment 

xl402B Laingsburg phosphate and 

lime 
xl401B .. .Laingsburg phosphate 

alone 
xl402 A . . . Florida Rock phosphate and 

lime 
xl401A . . .Florida Rock phosphate 

alone 

xl601 Bonemeal and lime 

xl602B . . . Bonemeal alone 



AGRICULTURAL VALUE OF INSOLUBLE PHOSPHATES 

JACOBUS STEPHANL'S HARMS 



PLATE 1 



^i 



m if IIS 




'^o^A fSOlS '502A 'SOM ^^04^ 




Fig. 1. 



i^sf ^ 












J ji"' '\ 




Fig. 2. 



401 



SOIL SCIENCE, VOL. XIII, NO. 5 



PLATE 2 

Fig. 1. Wheat on brown silt loam series showing the effect of urea on the availability 
of aluminum and iron phosphates. 

Fig. 2. Wheat on yellow silt loam series showing the effect of urea on the availability 
of the calcium phosphates. 



402 



AGRICULTURAL VALUE OF INSOLUBLE PHOSPHATES 

JACOBUS SIEPHANUS MARAIS 



PLATE 2 




Fig. 1. 




■L J| PH(BPHATE»C 



Fig. 2. 



403 



TLATE 3 

Fig. 1. Wheat and clover in sand series showing the best pot with each of various 
phosphates. 

Fig. 2. Buckwheat on sand culture showing effect of ignition on availability of 
aluminum phosphates. 



404 



AGRICULTURAL VALUE OF INSOLUBLE PHOSPHATES 

JACOBUS STEPHANUS M.4RAIS 



PLATE 3 







I m m P js ^1'' 

SALDANNAhIdUFRENITE J FLORIDA RDCK I ROCK PHOSPHATE i30.NlE MEAL ' '. &C1DF 




I 




I'IG. 1. 




Fig. 2. 
405 



PLATE 4 

Fig. 1. Roots of sweet clover clinging to the porous pots. Pot on right received 
soluble phosphorus. 

Fig. 2. Effect of sweet clover roots on smooth surface of wavellite. 



406 



AGRICULTURAL VALUE OF INSOLUBLE PHOSPHATES 

JACOB as STEPHANUS MARAIS 



PLATE 4 




Fig. 1. 







WAVELLITE 

ALUMINUM PHOS. 



Fig. 2. 



407 



PLATE 5 



Fig. 1. Effect of sweet clover on smooth surface of Laingsburg phosphate (rock 
phosphate). 

Fig. 2. Effect of sweet clover on smooth surface of Saldanah phosphate. 



408 



AGRICULTURAL VALUE OF INSOLUBLE PHOSPHATES 

JACOBTfS STEPHANUS MARAIS 



PLATE 5 



i^l?A ' \ 




^ 



Fig. 1. 




Fig. 2. 
409 



VITA 

The author of this paper was born June 6, 1896, at Pretoria, Transvaal, 
South Africa. From 1903 to 1909, he attended the pubHc schools at 
Malmesbury, in the Cape Province; from 1909 to 1912, the boys' school 
at Pretoria in the Transvaal;, in 1913, he attended the boys' school at 
Stellenbosch. In December, 1916, he obtained the Pass B.A. Degree from 
the University of Cape of Good Hope, and a year later, the Honours 
B.A. Degree from the same university. In 1918, he entered the Graduate 
School of the University of Illinois where he was a student up to June, 1921. 



LIBRARY OF CONGRESS 




002 756 358 2 



SOIL SCIENCE 

VOLrME 13, NUMBER 5, MAY, 1922 



CONTENTS 

D. J. Healt and p. E. Karraker. The Clark Hydrogen-Electrode Vessel and Soil 

Measurements 323 

Selman a. Waksman. Microorganisms Concerned in the Oxidation of Sulfur in the 

Soil: III. Media used for the Isolation of Sulfur Bacteria from the Soil. 329 

William Mather. The Effect of Limes Containing Magnesium and Calcium upon the 

Composition of the Soil and upon Plant Behavior 337 

J. S. Marais. The Comparative Agricultural Value of Insoluble Mineral Phosphates 

of Aluminum, Iron and Calcium 355 



